Stupidity: When Kellin is hit with the neurolytic emitter, it erases her memories of the outside world. However, the effects aren't immediate and take some time (enough to get her to sick bay and have time to talk to Chakotay). However, she spends her time telling Chakotay to help her remember their relationship, etc. when her memory is gone instead of just making a recording of herself describing the relationship and her feeling which can then be shown to her after her memory is gone. She might not believe the recording, but seeing herself discuss the situation would be better than just relying on Chakotay (who will soon be a stranger she never met) telling her something so unbelievable.
Living Hell - S1-E8
Stupidity: When the villain reveals to the doctor that he plans to transplant her brain into his own head, the doctor tells him it isn't possible because the brain can't survive outside the head. Forgetting to mention the fact that he will die the instant he takes his own brain out, making it rather difficult to complete the procedure, that is, if he didn't pass out from the pain of opening up his own skull first.