The Alternateville Horror - S4-E8
Stupidity: Colin tells the group he saw a ghost (a non-corporal shade) who walked through the door. They laugh at him and blame it on what he was watching on TV or too much sliding. Given everything the group has seen, vampires, a dragon, etc, they should have least checked it out. Quinn himself was once on the astral plane and nobody could see him except one girl who thought she was seeing a ghost, and he could walk through solid objects.
Stupidity: And the end, after sliding to another world, the group is caught on film by someone in her house. Quinn goes (illegally) into the home to get the tape. There is no point to this. Throughout the series, many people have seen them arrive, or leave, and usually when there's a mix up with one of their doubles, they spend so much time trying to convince people they are in fact from another world but can't prove it. Quinn doesn't even bother to see how long they're going to be on that world first.
Stupidity: Throughout the series, people in the group always say they need to stick together and only separate if absolutely necessary. Here, Quinn and Rembrandt take a vacation pretty far away from Wade and Maggie (they had to fly to their destination). And they do so knowing they have no way to directly communicate with the others (i.e. cell phones) in case something goes wrong, which it does. They get bumped from their flight, but it might have been easier to get Wade and Maggie to fly to them.
Data World - S4-E17
Stupidity: One of the hotel rules is "no singing in public places." Rembrandt ignores this and starts singing, despite pleads from the bartender. The group is already questioning how they got inside the hotel, why there's no charge but nothing is free, and Rembrandt even wants to know what the hell is going on. Rembrandt has been on enough worlds to see what happens when people break the rules. It's stupid for him to start singing in a place he knows nothing about as if this rule didn't apply to him.
Chosen answer: Quinn's brother was going to be a recurring character, so "introduced" in this case referred to the actor becoming a semi-regular.
Jean G