When Tom goes into the Impressionists gallery after the building is being evacuated, he slips under the closing gate, and runs for the painting that he steals... But on his way back, he has to climb over a rope gate (the ones they use to close off stuff) then he climbs under the gate, with the brief case holding it up...
I watched the movie two nights ago and specifically looked for this slip up. I am sorry but in the scene just prior to Tommy going under the gate, the real guards are questioning the thieves and if you look at the gate at that point, there is a rope and behind the rope is the briefcase that Tommy left there. That makes it the same gate and he would have needed to jump over the rope.