The Farting Sex Tourist - S1-E4
Stupidity: Joel finds prints in Serbian and is stressed because they'll never find someone to translate Serbian in Santa Clarita, goes to great lengths to interact with the principal's Serbian grandmother, etc. Google translate can understand Serbian just fine, but this doesn't even occur to him.
Suggested correction: Except the 16th century Serbian alphabet isn't available to most people to type into google translate. Plus the script used makes it difficult to distinguish which character is which sometimes even if you could find characters to cut and paste tediously into google translate.
Stupidity: The Seal team is sent to apprehend an HVT. Their insertion didn't require a long hike. Usually on long range recons where Seals walk 20+ kms and don't fix to get into a gun fight they don't wear plate carriers with ballistic plates due to the weight and extra body heat generated. However on this op their journey was short, and they knew they were going to get contact inside a structure where getting shot is more likely since it's close quarters battle, but they didn't wear plate carriers. (00:24:30 - 00:26:00)
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.