So, your application gets deployed in a week. It has gone through system, integration, and user acceptance testing and has passed all three. However, the final phase (in our case, a few weeks before deployment) is that actual end-users (a much larger group than those who perform UAT) go through a 'business transition' phase where they are all trained on the new system so that they will be good-to-go when your pride and joy gets deployed to production.

We received a call a week before the production deployment that popups weren't working on some of our end-user's machines, who were running IE 6.0.2800.1106 on Win2k. A side note is that the only browser that our web-based application is contractually oblicated to work on is IE. After finding a random computer in the corner that matched the Win2k and IE specifications of the user, we fired it up and gave our application a shot; naturally, we couldn't reproduce the error. A client visit later (where everything worked in Firefox, and a fresh upgrade of IE did nothing), we saw that the popup was actually being displayed, but it was blank and there was a javascript error on the 'parent' opener IE window:

An error has occurred in the script on this page.
Line: x
Char: y
Error: Class not registered

Going through the code, it ended up being a "window.open" call that was failing. Being unfamiliar with the error (and working knowledge of IE's use of dll's), some searching around turned up another blog where someone had a similar issue. They referenced the article below on Microsoft's support site, and it ended up being a single command line fix. For some reason, the urlmon.dll file needed to be registered with Windows. After running the following line in the command prompt on the 10 affected computers, all worked perfectly:

regsvr32 urlmon.dll

I don't know how many people this will help, but hopefully the ones that need it won't be left scratching their head for too long!

Microsft support article: http://support.microsoft.com/default.aspx?scid=kb;en-us;306831