I made a few, but my biggest was this:
My opponent played a Benalish Heralds and I panicked, but wonder of wonders, he attacks with it and lets me chump block before I can topdeck removal.
Unfortunately, he plays a second and I want to stall him before he can take advantage of it and bury me with card advantage.
So I play something and use my last two or two of three mana to play Barrin's Unmasking on it -- forgetting I had both Exclude and Probe in hand!
I draw Agonizing Demise next turn, anyway, but he eventually kills me by using Agonizing Demise on my 5/5 Ruham Djinn when I had complete board control and 5 life. Worst thing is he had no red in the deck and coincidentally had a land you could sac for red. I really thought for a long time if I could have kept Barrin's Unmasking in hand somehow, because I could've bounced a white permanent and made Ruham 3/3!
I am doing a local "prerelease" limited this Saturday-- I'll make sure to let everyone know what my mistake was. ;)
Mine was forgetting to sign up.
My opponent casted Agonizing Demise on one of my creautres to kill it. I responded by Recoiling it. I put the creature into my hand and wait for my opponent to discard. He's like "What's up?" And I say "You need to discard." "No, the card's controller does." So I had to discard the creature. I responded by saying "OK, that play was sub-optimal."
I actually played pretty well with the exception of keeping a dubious hand in the first round of the top 8. I did play a Quirion elves and name white when I had 3-4 plaind out and a blue apprentice out though( it didnt matter but naming black so I could drain would have been the smart play).