Auto-update has installed nightly build instead of WP3.9.1

A weird thing seems to have happened with three of our sites overnight!

All of them updated automatically from 3.9 to 3.9.1. However, three of them then updated again, a little later than the rest (between 20:38 and 21:35 UTC) and updated to 4.0-alpha-20140507 instead).

I’m not sure why this has happened – as far as I can recall, I didn’t do anything different when originally installing the three offending sites, and the original updates to 3.9.1 happened in the middle of the sequence of our other sites’ updates!

Can anyone shed any light on this, please? I’m off to try to roll back the later update now, but I’d rather avoid it happening again if there’s something I can do to prevent it!