Thought this would make for a funny thread.
At JSS I corrupted my opponent's shard phoenix. If I had cast it on him I would have won.
Recently I tried to go off with upheaval/infestation. He floated to counter my infestation. I floated in response, then tried to Actions his counter with no land in play 
At Reigonals I was in a race with a control deck. I was playing sligh and had sided in red elemental blast. All my friends were standing around as I needed this victory to make top 8. I was holding a REB. I cast Boil, he countered, I let the counter resolve. WHY?? I still don't know. My friends were practically choking to death cause they couldn't tell me what play to make. I had the untapped mountain...I just completely forgot about the REB.
Didn't make top 8 
By Henge Wolf (Wolf) on Thursday, January 17, 2002 - 02:30 pm: Well, the other day I had a Shivan Dragon in play, with a Berserk AND a Fork in my hand. For some reason, I felt charitable and decided to wait another turn to cast them for the win. On his next turn, he casts double Ball Lightning and kills me! It was my little brother though, and I'd already Berserk/Forked him to death twice earlier that day. The moral: mercy gets you nowhere in magic.
Way back in the day, when I was still new to the game, I channel/fireballed a guy who had two untapped Tundras in play. For some reason, I remembered Tundra produced white, but forgot it produced blue?!? Of course he countered my fireball and I was so embarrassed I cast Lightning Bolt on myself! That was actually at a tournament, but in my defense, I was playing someone elses deck that I wasn't at all familiar with. The tournament organizer let me play for free if I'd use his deck and try to take a couple people out. Those were the days before sanctioned events were common.
-HengeWolf
By Aragorn on Thursday, January 17, 2002 - 03:35 pm: This about a year ago. my opponent was playing the MIshras helix version of wildfire w/ ensaring bridge i was playing old type 2 stompy, with cradles and scrolls and the like. each game i got him down to 1-2 life then he would get control. so at the beginnig of each turn he would tap my lands, but what i could've done but didnt was cursed scroll him in reaspons to him tapping me out, I could've won Damn it!
By Gzeiger (Gzeiger) on Thursday, January 17, 2002 - 05:31 pm: It's probably a good thing you let that counter resolve, as Red Elemental Blast is only legal in Type 1, which as I recall was not the Regionals format...
By mdtrimm on Friday, January 18, 2002 - 01:27 am: i got you all beat. i lost to camel deck with a fully powered keeper deck a few weeks ago.
By MojoJojo on Friday, January 18, 2002 - 03:57 am: Two years ago, I was playing a kinda Green control deck (elves, Hermit, Masticore, etc) against a Blue Accelerated deck.
He was mana screwed but managed to drop an Energy Field to gain some time.
After a few turns of both of us drawing and doing nothing, I drew at last a Creeping Mold.
And I Mold the ... Energy Field (instead of the land which would have triggered the Energy Field sacrifice).
My opponent got his third land, played Hibernation, then got a monolith, went Treachery + Treachery and I lost miserably.
It was a pretty important tournament too...
By Lava on Friday, January 18, 2002 - 06:47 am: I have a bad habit of some times forgeting to playing anything on the first turn this is particuly bad when I am playing sligh and could have droped a 2/1, it often costs me the game.
does not effect me in toraments though since I only ever play mono blue, no first turn drops there...
mdtrimm - that sounds bad... (I want to beat keeper with a camel deck)
By General Talon on Friday, January 18, 2002 - 03:15 pm: Never really made this play, but I thought about it, just to be stupid. 1st turn: Entomb Legacy Weapon!
Yesterday I was playing a guy named Kretzschmar who uses a really fast Type 1 B/R turn 2 or 3 killer bye entombing large creatures and exhuming them. My Type 2 U/G Time Fields slowed him down so much that by around turn 20 he had nothing but a Shivan, and I had a Living Airship. Anyhoo, he finally brought out his Dreadnought with an Unearth, with I neglected to Snake because I could always Repel it and Snake it later. He was at 3. I was at 5. I attacked with 3 Gaea's Skyfolk for the win, and he played that instant where you bring a creature from your graveyard and hand into play and switch them at end of turn, pulling out a Bloodfire Colossus. I missed this altogether, so I didn't Snake it. He blew the Colossus and won since it was his creature that killed us both.
By Matt Caplan (Caplan) on Friday, January 18, 2002 - 03:56 pm: General Talon - both of you screwed up, rules-wise.
Phyrexian Dreadnought
Color= Artifact Type= Artifact Creature Cost= 1 MI(R)
Text (MI+errata): 12/12, Trample. ; If ~this~ would come into play, sacrifice any number of creatures with total power 12 or greater instead. If you do, put ~this~ into play. If you don't, put it into its owner's graveyard. [Oracle 2000/02/01]
The Dreadnought would never come into play, as he only had a Shivan to Sacrifice.
102.4. If both players lose simultaneously, the game is a draw.
It doesn't matter whose creature it was.
By Paranoid Android on Friday, January 18, 2002 - 11:37 pm: Sometimes it pays to read the wording on the card. There was once when I played a kid at a card store, I was playing sligh and he was playing some blue/green deck. He had a thawing glacier in play and I wastelanded it. In response he played the thawing's ability and return the thing to his hand. it was very much later before I realised that thawing only returned to its owner's hand at end of turn!
By Puschkin, Defiant Vanguard Against The Phyrexian Invasion (Puschkin) on Saturday, January 19, 2002 - 07:46 am: I once sleighted a Anarchy to white!
By JP 'Polluted' Meyer, the Archivist (Jpmeyer) on Saturday, January 19, 2002 - 01:34 pm: I was playing a casual game with a friend. To see who went first we cut our decks and looked at the bottom card to determine who went first. My bottom card was Sengir Vampire...which I promptly Consulted for and lost my whole deck.
By General Talon on Saturday, January 19, 2002 - 03:13 pm: Caplan, you don't read much errata, do you?
Dreadnought's ability has been changed to an additional cost, so exhuming it into play doesn't trigger it.
When two players lose simultaneously by damage, the controller of the damage wins. This isn't errata, it was ruled by the only judge in my school and his ruling hasn't been overturned yet.
By Shadow (Shadow) on Saturday, January 19, 2002 - 03:26 pm: Talon, are you joking (I'm sometimes bad at detecting sarcasm)? Those are both definitely definitely definitely wrong. The judge at your school was wrong. Check in the Rules Mill.
By pseudosoldier on Saturday, January 19, 2002 - 09:33 pm: Today, I didn't float the mana for a Rushing River when my opponent cast Balancing Act and left mana floating (for Terravore).
By Elrond, the High Priest & Pokemon Slayer (Elrond) on Tuesday, January 22, 2002 - 05:51 pm: Once, in extended I was playing my old Ophidian deck against rebels. The guy had 1 creature and I had 2 and he has Planar Collapse (the one that triggers at 4 creatures if that's the wrong name) and I would have only had to attack one more tima to win but what do I do in my first main phase? Play my third ophidian.....
*tries to kick himself in the head*
By Rico Jones, Elfman Extraordinaire (Rico) on Tuesday, January 22, 2002 - 06:16 pm: Doesn't that card only trigger during it's controller's upkeep?
-Rico
By Etrigan on Tuesday, January 22, 2002 - 07:11 pm: I think the one thing I am most guilty of is playing instants in my turn when they should have waited till my opponent's end of turn. For some darn reason, I keep thinking Fact Or Fiction is a sorcery. It should be a sorcery, but it ain't
By Elrond, the High Priest & Pokemon Slayer (Elrond) on Tuesday, January 22, 2002 - 08:12 pm: I don't remember exactly how it happened. That's the basic reason why I lost, tho. I musta played it after my attack
By BWM, phyrexian infiltrator on Tuesday, January 29, 2002 - 09:35 am: I've once mana-drained a Draco (on apprentice), what do you do next turn with 16 mana in your manapool floating around...
By Erik (Erik) on Tuesday, January 29, 2002 - 11:05 am: torch? geyser? stroke? morphling? die horribly?
By BWM, phyrexian infiltrator on Tuesday, January 29, 2002 - 02:42 pm: He played an Orim's Chant (and I had a hand without any instants that cost 16 mana...)
By Random-Miser on Tuesday, February 12, 2002 - 02:39 am: guys?
I AM GAY!
MY GREATEST MISTAKE WAS BEING MADE! ALSO I MADE MY CRAPPY DECK! I ALSO THINK THAT FORCE OF WILL IS BAD CUZ IM GAY!
By folgerty on Tuesday, February 12, 2002 - 03:16 am: the most recent really bad mistake i've made is that i blanacing acted while floating like 7 mana and forgetting to ask the all important question "cards in hand?". wow...did i ever look dumb discarding my entire hand to balancing act and burning for 7. WOOT!!! oh, and at the torment prerelease my opponent was playing b/r/g and had 1 mountain, 1 forest, 1 mossfire valley then a pile of swamps and i rancid earthed his mountain. god only knows why. next turn he play gurzigost and i lose horribly. WOOT!!! i am the captain of the scrubs!!!
By Nathan C on Tuesday, February 12, 2002 - 10:46 am: Hey, I've got a pretty bad one that you'll like. It cost me a GAME in a TOURNAMENT, too. I was playing a U/W/G/B Control Deck vs. a Necro player. He was at ONE life with a Wall of Souls out. I had two Elephants out from Call of the Herd. All I had to do, was wait for any of the FOUR swords to plowshares in my deck (plus gaea's blessings to get them back) and swords his wall. No, I Stupidly declare an attack (forgetting I am only at THREE life).
By Nathan C on Tuesday, February 12, 2002 - 10:47 am: He had Necro in PLAY, by the way, so he could never draw.
By Superman on dope (Dope) on Tuesday, February 12, 2002 - 03:44 pm: My stupidest play ever was playing against a Domain deck with BBS before Jan. 1. He had all land types in play and I cast Fact or Fiction at the end of his turn. In response, he cast Evasive Action, and I must have misread the card to read all basic lands 'cause I hard cast a FOW to counter it when I could have just tapped 5, and he responds with another Evasive and I'm tapped out! I could have forced it through with that FOW by chucking a card if I had just paid the 5 colorless.
I ended up winning the game anyway, but that sure was stupid.