Hey, it's that guy! And he's playing for charity! (Producers who value their handiwork should not use this full-time on a game usually played by civilians. It's been tried several times, and on every occasion the show in question was canned within a year.)
Wow — you didn't even get one answer right in the bonus round. (Not explicitly a game show trope, but occasionally appears and sometimes combined with What an Idiot, where a contestant gives an obviously bad answer, if they're able to even answer — e.g., "Man, that was such an easy puzzle; I can't believe you missed it that badly!")
When game shows collide with fiction. Usually makes no difference in the characters and keeps the Status Quo intact, but occasionally will set up a plot thread or change a character for the better (or worse).
"It's clear you're struggling today, there's too much of a difference between the scores that it's now impossible for you to catch up ... so why don't we end things right now and award the game to our champion." (Not specifically a game show trope, but happens on occasion due to unusual circumstances regarding poor performances that the game ends before its natural conclusion, most notably in the Pyramid franchise.)
"Who is the Father of Our Country?" followed by contestant drawing a blank, and then being dumbfounded when the correct answer is given. (Certainly not limited to game shows, but plenty of these moments have led to game shows' all-time funniest or greatest moments. Goes on the YMMV tab for the specific shows.)