My absolute favorite was a glitch in the cpu for WC 2010 DS game. If you used Mystical Space Typhoon on a face-down Solemn Judgment, the computer opponent would sometimes chain the Solemn Judgment to negate Mystical Space Typhoon. Always made me chuckle.
For actual human players... worst I've seen was someone who totally face-palmed in a horrid misplay. Opponent summoned TGU on turn 2 (my friend went 1st) while he had a face-down Solemn Warning and an in-hand Maxx "C". For some unknown reason, he allowed TGU effect to go through, which summoned a Broww and didn't use Warning until after his opponent tried to summon Zenmaines. Well, opponent did a Foolish Burial --> Grapha and then went ahead and summoned Dark Armed Dragon, put down Gates, used it to discard Beiige (summoning him), drew a card, brought out Grapha, did more horrible trickery, and ended up OTK'ing him. Had he simply simply used Maxx "C" in a chain to TGU, opponent wouldn't have had any DARKs in the grave for DAD, wouldn't have been able to use Gates as easily (since the only thing he could banish would've been whatever he sent out with Foolish Burial), and all my friend would've had to deal with was a Zenmaines on the field, while gaining at least a +1 off of the Maxx "C". Additionally, he also could've optionally just used Warning when TGU was 1st summoned, which would have perhaps been safer than the above and would've prevented the OTK. Made me sad to think that "this person" was someone who I was actually friends with. =/
