Thanks to all :)

I thought it might be something to do with the browser caching the script. But as pointed out by sgifford, the browser only caches the result.

So it's probably the script. I did more testing but couldn't nail down the bug. Basically it's to do with the value of a variable that is inserted or updated into the database. This value should not exceed a certain upper bound. But for this user, it had exceeded that maximum value.

My tentative solution is to alert me with an email when the upper bound is exceeded and to re-adjust it for the user before it's added to the database.