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.