The Clean Room Infiltration - S8-E11
Stupidity: On the way home from the university after wiping their names from the sign in sheet, Leonard, Howard, and Raj are all singing Jingle Bells. Leonard is driving and has his eyes closed all the way through singing the song, this makes no sense at all giving he should be looking where he's driving.
First Kiss - S1-E2
Stupidity: When Jeff Malone leaves the sandwich shop, he leaves the sandwich on the table. It would have been very easy for them to tell what was on the sandwich without needing to wait for Max to get home to recreate it. (00:02:41)