Even the most obvious assumptions aren't all that obvious. Who believes that a software tester tests a new release in the production environment! This has actually happened. When a new release was installed in the acceptance environment, the acceptance tester was signalled.
When he finished testing, a whole list of issues was reported via the issues tracker. We had absolutely no idea of what was going on until we asked for the URL and user accounts with which the tester had tested. The URL that was used was actually the URL of the production environment! After that it was totally clear to us what went wrong.
Lesson learned: even if your assumption is obvious, do not assume!