"User name cannot contain spaces".
Familiar error message? Yep, but this time-wasting step would be unnecessary if some website designers would just engage brain and spell out on their requirements on the registration page. To add insult to injury, the sites which are thoughtless enough not to give you the info you need up front will often also clear the whole form you've just laboriously filled in, to boot - not just the one field which you haven't completed exactly the way they want (if you're using IE, at least - "Back" on Opera often preserves form contents and has saved my bacon on many an occasion).
Why oh why don't websites requiring registration tell you at the start, on the sign up/registration page, how you're supposed to fill in their forms?
If I had my way, an Improbulus Edict for all websites requiring registration or login would be:
Specify your form criteria on your sign up page, including (1) the minimum requirements, and (2) what isn't allowed, for both user names and passwords (and things like what format dates must be in, and whether passwords are case-sensitive). In a smaller font, if necessary, but don't expect users to guess your invisible conditions!Breach of which ought to be punishable by an Improbulus flogging. Though maybe that wouldn't be considered punishment by some.