The client can be modified to allow characters to move in ways that are impossible with the normal client. All of those modifications are actionable of course. The client does check itself to see that it's intact but people have ways around that.
You can do things like induce lag on the client to allow you to keep flying briefly when you enter a no-fly area. That's actionable too but unlikely to get you a ban unless you do it in front of a bunch of people, because it does occasionally happen in normal play.