In addition to the frontend issue mod mentioned, it often happens accidentally without any errors or warnings when using a VARCHAR in a relational database, which have a maximum length. If the username field is VARCHAR(20), the application ignores database truncation warnings, and the developer didn't think to check the username length before storing it in the database, it'll truncate a 21-character username without you knowing. This comes down to the devs using sensible field lengths and handling edge cases.

I think mysql is the only database that auto truncates varchars isn't it?

Well, you have to have some limit. Otherwise a user could register with a 1GB username. This might break all sorts of things that assume they can display or work with usernames.

This is why you prevent them from entering one that is longer than your limit. You don't let them enter it and then truncate it...

To keep username from ruining the front-end.

If I had a 2,000 character username, this page would look really stupid.

I saw an example of that on a JavaScript-related site recently, where a guy's username was aaa...aaa several hundred characters long, causing a ludicrous horizontal scroll bar. You'd think it would be easy enough to say upfront during account creation that both usernames and passwords are limited to x characters.

That is what this css is for:

  overflow:      hidden;
  text-overflow: ellipsis;