My most memorable one wasn't technically complicated, but we were receiving mixed feedback on a site that I had worked on. Some of the test group though it was

My most memorable one wasn't technically complicated, but we were receiving mixed feedback on a site that I had worked on. Some of the test group though it was working fine, the other was having "doesn't work" issues. It turned out that the problem came down to a single dialog box, when people tried to cancel a process on the page. The resulting dialog said: "Do you want to Cancel?" "OK" or "Cancel" Rewording the options fixed the issue...