In the scene where Alan, Lex, and Tim are climbing the fence.

Alan and Lex are on the ground and Tim is still climbing, the alarm goes off to signal that the power is coming on. When it goes of we see a closeup of Tim lose his footing. Then there's a shot of Alan and Lex telling him to jump, in that shot,on the left side I believe, you can see Tim lose his footing again.