It's entirely possible that, during the process of de-partial-stating, we discover that we accepted some piece of state which we should not have done. (Simple example: it may turn out that the sender of a piece of state was not actually in the room at the time). We may have told clients about that state, and we may even have made other event-auth or access decisions based on it.

(I think this is also possible for backfilled events even in the non-faster-joins case, though a bit less likely.)

We should figure out the ramifications of that, and do something about it.