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.
Character mistake: When Eric is explaining the bomb to Sheila and Joel, Sheila says to push the detonator when 50 feet away. Joel says no 50 yards. Eric says that is 500 ft. It's actually 150 feet. 3x50=150. (00:23:35)