This small check just ensures that we aren't about to blindly accept that the calling code knows what it is doing. There are some unknown cases where NewRoom gets fired for rooms we already know about, so in those cases we just change it to a PossibleTagChange which is what the caller likely intended.
Many of the edge cases are unknown, though this can happen for an invite being accepted (for example). It's easier to handle it here instead of tracking down every single possibility and fixing it higher up.
Travis Ralston
da2fd35094