Sometimes the test you do.... is not the test you intended to do.
Case in point:
Today I went to do a simple upgrade test. On a given platform in a given configuration, upgrade from the most recent released version to the new version that's coming out. Simple.
I completely forgot that we'd tested a "power event" on that system earlier this morning. (Note: for "power event" read "a tester went in and pulled the plug on the power strip for that entire system"). The system handled the "power event" just fine, but I hadn't yet gone in and completed the products startup validation process.
So my system was not in a good state when it started the upgrade. And the upgrade failed exactly like it was supposed to.
Not exactly the test I intended to do, but it was a perfectly valid test. So what's the moral of the story?
If you're attempting at test, and it does something unexpected, take a quick look. You may have done a different test.
Now off to do the test I meant to do originally!