Someone is unaware/doesn’t care, so they sit in your seat before you get there.

You arrive and ask them to move, they apologize and say “oh but my friend is here, I see some empty seats up there you could take instead!”

So you go sit in those empty seats, hesitantly. Then someone comes in and tells you that you’re in their seat. You try to explain the situation but…

Etc etc etc.

This has happened to me multiple times on long-distance transit before (bus/train), it ends up being chaos if just a few people don’t perfectly follow the assignments.