|Don't ask to ask, just ask|
Is there some spooky content command or something to tell a user's browser to not cache files or something like that so this doesn't crop up for other users?
these HTTP header values may be relevant
another method is to make sure the URL is different each time (e.g. by appending a GET parameter showing the current datetime).
update: don't forget, the user's browser isn't the only place that caching can occur. it can happen at your end, at the user's end, or anywhere in between (e.g. reverse caching proxy at the ISP)
In reply to Re^3: How to wait until GD Image it written, then continue