Krillin: It must have been when he fought Dracula Man, and no matter what he did, he screwed up in the fight. And that after boasting about how strong he was.
Goku: A tie between his bad judgement over Gohan's will to fight Cell, and when he begged for help against Android 19.
Tenshinhan: Apart from the belt-trick Goku pulled on him in the 23rd Tenkaichi Budokai, it must have been when Goku saved him for a 2nd time against a villain, when he had given his all.
Piccolo: It has to be almost everything that has to do with Gotenks, really.
Yamcha: Oh, boy, where to start. We have to pick between:
- Getting a tooth punched out by a kid
- Seeing Bulma naked on two occasions
- Getting his ass kicked by a mummy (after boasting)
- Losing in any chance he has got in the finals of the Tenkaichi Budokai.
- His death by the hands of the Saibaman
- Him getting cold feet when he heard of the Androids.
I think that I have to go with the 4th point there, with his fight against Shen as a clear winner.
Bulma: When she showed her p*ssy to Kame Sennin.
Yajirobe: When he tried to become a friend of Vegeta in the middle of the fight. And also the senzu bean incident.
Karin: He stated that he knew of everything, just to be told that Shenlong has died. Knows of everything my ass.
Kami: When even Goku could figure out that his plan to kill Piccolo wouldn't work without Kami dying too.
Vegeta: Almost all of his fights, really.
Freeza: When Krillin sliced some meat out of him with his Kienzan.
Cell: Getting kicked in the head so hard that he puked.
Buu: Pretty much his whole fight with Gotenks.
