The Power of the Doctor - S13-E9
Stupidity: After the crew of the space train have taken out the Cybermen, the Cybermen begin regenerating, which one of the crew points out. This guy seems to be aware of how regeneration works, but doesn't shoot at them again. In the series, it has been shown that shooting someone during a regeneration cycle will kill them instantly, but this idea is never considered.
Recognition - S1-E17
Stupidity: After fighting Wootox in Sky's body, why didn't Cruger contact the other Rangers and inform them about the body switch? This would have spared everyone a lot of trouble.