At the beginning of the movie, when Dennis Hopper holds the hostages in the elevator, you can hear him snap his fingers. Later on in the movie you can see that he has no thumbs! Well, it's hard to snap your fingers without a thumb...
You forget - they even mention in the film that he has 9 fingers and thumbs. Close but no cigar.
The actual Slip up is this:
I don't remember the exact point of the film, I think it's just after
when Howard Payne's (Dennis Hopper) house is blown up, but it's
definitely before they rig the video on the loop.
Jack and Howard are talking on the phone about negotiators. We can
clearly see Howard's right hand with all fingers and thumb holding the
phone. Suddenly, Howard clicks his fingers with his other hand, which you
surely need a thumb for?
So how does he click his fingers when they make a good point of saying
he only has one thumb?
Ah well.
At least I didn't mention the jump...