Revealing mistake: When Alex drops the cigarette lighter to ignite the gas, you can see the flame is started from a different source, as it starts on the other side of the screen, a fair distance away from where the lighter landed.
Revealing mistake: When Alex is about to peel off his burned skin to reveal his iconic "crow" face, you can tell where the makeup switches from attached prosthetics to the fake "pull away" makeup.
Revealing mistake: When Alex sees the arm in the burning rubble of the club, you can tell the arm is a prop because it is melting in spots, not burning up like real skin does.