Stupidity: On 8/12/04, Hope specifically told everyone that when the coin was thrown into the force field, there would be a bright flash and that they shouldn't look at the forcefield when this was done. But all three times that the coin was thrown at the force field that day and the next, everyone looked right at it the whole time, and then recoiled in pain at the flash that inevitably occurred.
DavidK93
The Night of the Firebrand - S3-E2
Stupidity: West is introduced to, then knocked unconscious by Vixen as she hides a club behind her back. A trained Secret Service agent like West would not have let his guard down in this situation, as the entire fort was awash in bad guys.
Scott215
Stupidity: The plans Hochstetter and Burkhalter find in Hilda's desk turn out to be for the Hindenburg, a large dirigible. Burkhalter says he never saw the plane (sic) they show before. The Hindenburg was a German propaganda icon and very well known to every child in Germany. It's nigh impossible a Luftwaffe (Air Force) general would not recognize plans of a dirigible when he saw them, or not know the Hindenburg. Note that despite everything, the plans from Hilda's desk are probably meant to be not the same ones Carter later says he found inside the boxes - A Luftwaffe general not recognizing the difference between a jet plane and a dirigible is even more stupid. To avoid unnecessary corrections: Yes, according to the boxes the kits aren't for the Hindenburg but for JU-87 Stuka bombers, but that's a different mistake and not relevant for this one.
Doc