I actually played pretty tight this season, although I am sure I am forgetting some epic boneheads. I did throw a Giant Mantis into an obvious triple block to trade it for a 2/2. That was bad but not atrocious.
Here is one from last PR match (having been playing magic for a good 6 hours straight). Match 3 game 1, I am way ahead on life.
His board:
a 3/2, a 2/1, a 2/3, 3/4
seed guardian with 3 creatures in graveyard. No cards.
My board
Ruin Processor 7/8, 2 2/3 eldrazi and a 1/1 scion, plus
Stoneforge Masterwork.
In hand I have
Flaying Tendrils, and plenty of mana. I think about playing Tendrils, but didn't want to kill my scion because of the masterwork, and my 2/3s became 0/1s. I thought about playing it post combat but that didn't make any sense. I end up attacking with just a 10/11 because I miscounted his power as 10, not 11. So I trade my 7/8 for his board... but he got a 7/7 elemental token.
Luckily, I had 20 life and enough chumpers to wait until I top decked a removal spell. Still, turned an easy win into a nail biter.
If I play tendrils pre combat it *exiles* his weenie and gives him a 0/1 and a 1/2 against my 7/8 (and 2 0/1s). And if he chumps with the guardian, it's exiled and he doesn't get a token.
In my defense, I was very tired and don't count that well anyway. The next game I accidentally attacked with culling drone that was pumping my masterwork and it got eaten. I literally just tapped it during attacks accidentally.