This came out of the Philforum Focus Group thread.
Theaetetus had said that topics that had been tagged with "Philosophy" were disappearing from the Philosophy forum; that a given topic would only appear in the forum if the "philosophy" tag were one of the top five tags (the ones that appear to everyone under the topic title). That it didn't matter if it had been tagged "philosophy" at some point if "philosophy" was not currently one of the top five tags. That seemed unlikely to me.
I created this topic to test that idea and found that in fact Theaetetus was correct. (You weren't around at the time and it was easily testable.)
I tagged this topic "test," and it appeared in the "test" forum... but when I crowded out the "test" tag from the top five (not removing it entirely, just crowding it out from the top five), this topic disappeared from the "test" forum.
(As in, it's been figured out.)