When Mr. Tiny tosses the Cirque du Freak flyer out of his car window, the date on it when Darren picks it up says April 13th, yet when he looks at it in class right after, the date is September 13th. When the flyer shows up at Darren's house with Mr. Tiny's message in red ink, the date is back to April 13th.
At the very beginning, when Steve puts a rose on Darren's coffin, there is no spider bite/bruise on his right cheek. Later, when Steve is shaking Darren in the coffin (obviously before the graveyard), the bruise is there.
When Darren is about to drink Rebecca's blood, she pulls her shirt down and there is a strap still left on her shoulder. When the shot switches to her back, the strap is gone, and upon snapping back to her front, it is there again.
In the first show, just as Mr.Tall is introducing Madame Truska, as he says "...the exotic...", you can see more light passing through the tips of his unnaturally long fingers than the rest of his fingers, revealing that the actor is wearing hollow prosthetic to make the fingers appear longer.