So before you sneak into a section, you check if the usher is looking for tickets at the entrance. If he's looking at tickets, you don't sneak into that section.
Now, if you are sitting in a seat that isn't yours, and the usher asks to see your ticket, that is when you get up and move to the other side of the ballpark. You can say something like "oh, this is 108, not 109?" or "uh, I left my ticket in the bathroom, hold on", but you just want to get out of there ASAP.