I get this a lot, although since we are using GP over Citrix, there are more things that can fail or be killed.
I know that sometimes we have users that just get impatient and if their SmartList is taking more than 30 seconds, they give GP the 3 finger salute. As a result, their session just remains on the Citrix server grinding away until it finishes, then sits waiting for the user to reconnect. Then they start whining that they can't log in.
We also have a few issues with modules that contain customizations that aren't quite rock solid in the error handling department.
We also have an overnight process to clear dead sessions, since half of the time, the people forget they were logged in and just shut down for the night without logging out.