We have noticed that, for instance it is possible to type in a username ending in a space into the assign issue field.

There is no validtion on this, and as a result we have issues with spaces at the end in the DB.

This causes no end of pain, expecally when you use external user management.

We solved this by trimming the username (see: http://jira.atlassian.com/browse/CWD-1816)

however, a better fix, would be to force jira to valida/clan usernames for all username fields as they are entered.