The time on Make Your Move when the contestant wouldn't make a decision and Bob had to play the game for her. Looking back on it, I wonder if the lady was unwell.
I think this particular contestant just really didn't get what she was supposed to do, but your post about being unwell reminded me of another contestant.
Does anyone remember Zarie from the May 3, 1976 episode? It was rerun on GSN back in the day and I have the episode in my collection. Anyway, she was an older woman who was called as the very last contestant of the day. She was seated toward the back and took forever to come on down. She had to be nagged for her bid and unfortunately got onstage. She played Poker Game, or rather she just randomly chose prizes (with heavy prompting from Bob) and lost. Thankfully, she didn't make it to the showcase.
Seriously, this lady seemed really out of it. She had a blank expression on her face and seemed like she didn't really totally know where she was. It wasn't really funny or even terribly infuriating, just kinda sad. It legitimately appeared to me like she was in the early stages of dementia. I don't have my DVD handy to watch it again and explain everything, but anyone else who has seen the show can attest that the whole segment is...awkward, to say the least. Sorry to be such a downer, but that immediately came to mind.
As far as bad playings that were entirely within the contestant's control, I'll never forget the infamous Joy who played Ten Chances in 2006. She was far from a joy to watch and really kind of got lucky when she finally won the car.