I've seen someone not be able to use the "/tp" command (it would give an error message) because it had a player name in it with "é". I do not know if it is only limited to this command.

I saw this when I was at school, so it could be the case that the account was made using an unconventional method because this person originally had no Minecraft account. I also don't know if it is a hacked version of Minecraft or not.