I'd say I'm probably the biggest offender in our playgroup of stupid ridiculous mistakes. I should absolutely know better but I'm an idiot.

One time I was playing GW Maverick against POX, and I like to play 1 Mystic Enforcer instead of Thrun for the GSZ toolbox. It always has thresh when you search for it and it can win games that Thrun can't - plus it's just confuses the crap out of people. Anyway, I have a noble down, 3 land, and a sylvan library. I'm getting beat down by a 5/5 black demon token from that one sac land I can't remember (he floated mana, pox'd, then sac'd the land to make the flyer). I have one turn left to live, I the Enforcer in hand and I'm searching my top with sylvan for that 4th land don't find it and concede. My friend looking over my shoulder asks me why and I tell him I didn't find the land for Enforcer - he replies "why didn't you tap the noble?!" - I forgot that noble hierarch ... taps... for mana. Yeaaaa

Another time I was playing GW mav against MUD Stompy. He has a metalworker equipped with greaves and I have a hierarch and a qasali pridemage out. I have 1 stp and 1 tariff in hand in case he drops Blightsteel for the win. I look at the board and think, nawww he doesn't have it. Proceed to tap out play Elspeth, bless the pridemage to put him at dead next turn. He untaps plays blightsteel and wins...