If you're on desktop with JS enabled and ignore the error-like warning [1] presented to you when you try to create a username with a hash in, everything including and after the hash is silently truncated from the username. This means the user ends up creating a username that is different from what they typed in, which is suboptimal from a product perspective.
It should simply not be possible to create such usernames, and there should be an error presented to the user if they try.
[1]: It's not actually an error, it's just styled exactly like one in the user interface, probably to disincline people from trying to create it.
Version: unspecified
Severity: minor
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=64959
https://bugzilla.wikimedia.org/show_bug.cgi?id=64409