Such Sweet Sorrow: Part 2 - S2-E14
Stupidity: The Admiral stays behind to manually pully the blast door lever, sealing off the torpedo from the rest of the ship but dooming herself in the process. Thing is, the lever is right next to the door, takes minimal effort to pull down, and the door moves pretty slowly - they could easily have wrapped a shirt or something around it and pulled it down from outside. Or even taken a gamble, yanked it and dived under the door. Instead she just locks herself in and waits.
Through the Valley of Shadows - S2-E12
Stupidity: Burnham and Spock find one person alive from the otherwise-dead crew, miraculously. They know Control can infect humans, they know how advanced it is, the entire setup screams "trap", and yet they don't even question whether Gant even might be under Control's...well, control. A simple scan would have turned up anomalies and they could have avoided what followed.
