I've not had anyone snake my seat in this manner, but I have sat next to people who've tried to pull this, usually in domestic F.
Sometimes during the pre-departure service the F/As will go around and check people's boarding passes for whatever reason. They get to my row and I cheerfully hold it up/hand it over and my seat opponent will glare at me because they know the jig is up and steam back to 23E where they belong.

JD