If you happen to delete a User record from the user_base DB table, then try to views an Issues votes, or watches where that user was one of he watchers or users, then you get errors and stacktraces in the logs.
In order to handle external user management systems deleting users that are watchers and/or voters, we should be able to handle the user details disappearing, and just display the username without stacktraces.
Note that normal use with JIRA user management is NOT buggy, because deleting the user through admin page will delete the appropriate user_association records and update the vote counts accordingly.