Originally posted by Estyn:

I've had this happen before on both stone and wood structures. There seems to be no discernable reason for the single piece to collapse... fully supported, proper alignment/snapping, etc. Go away for a fairly lengthy period, come back, that lone piece will be broken again, even though everything else is fine.

Perhaps there is some internal non-visible issue like an old landscape artifact, invisible rock, etc or something that's interfering. I've never been able to solve it.

Yeah I suspect the same, or some internal data structure gets into an inconsistent state. I replaced the single 4x2 with four 2x1s, and it hasn't happened again since. If it does I'll mention it here.