The [Office] 2016 issue turned out to be a bug in sqlite. I don’t know if the bug was in Apple’s fork of it or in the open source codebase itself. There was an unfortunate misinterpretation of a struct between some 32-bit and some 64-bit code, such that the system ended up waiting on a semaphore with a multi-billion-second timeout. If you could wait a few thousand years the timeout would trigger and the app would un-hang, but unfortunately our hang detection code refused to wait that long and would presume the app was dead.
Of course, if Microsoft released the new Office apps as 64-bit rather than 32-bit (yes, they really did) then this would never have have happened in the first place.
Via Ars Technica forum.