As we start traveling to our sales meeting in the next several weeks, let's review some luggage etiquette. While many find it annoying to see that their locker is full when they get to their plane seat, overhead bin space above your seat is on a "first come, first serve" basis. People really don't understand, or they think that they automatically get that spot, since there aren't official rules that dictate who gets to use which bins, and thus leaving it open to interpretation & argument.
So while it may be annoying, particularly when someone has room above their own seat and still, for reasons unclear, has opted for yours, or when you're invariably forced to check your carry-on because there's no more space on the aircraft, you aren't entitled to anything. Further, it is never, ever okay to move someone else's bag.
Remember, rearranging bags and finding enough space for everyone's luggage was the flight attendant's job. Therefore, the cabin crew has the final say on luggage placement for safety & convenience.