Continuity mistake: The assassin rips out a magazine page with Leila's photo on it and sets it ablaze with a cigarette lighter. The face in the picture burns while he's still holding the page. But in the next shot, after he drops it, the face is still intact and starts to burn all over again.
Continuity mistake: When Darien reaches into the glass spider enclosure to retrieve the note, the number of spiders sitting on the paper changes (too quickly for them to have crawled away and back naturally). First there are two on top of the paper, then only one, then instantly two again.
Plot hole: In the pilot, it is established that the surface of anything/anyone made invisible by "quicksilver" is freezing. (Darien gets caught in the women's shower because the mist freezes on him. He freezes the beer mugs for the doctors.) But in later episodes, nobody notices him standing invisible - and freezing!? - next to them.