So, for a long, long time, the boolean operators on <mt:Entries> were case-insensitive; for example, "foo or bar" worked just as well as "foo OR bar". Sometime between 5.12 and 5.14 (which I finally upgraded to last night), this behavior changed, and now only the documented "foo OR bar" mechanism works. This is fine, and probably a good thing.
What isn't fine is the fact that I never had any warning about this; the old behavior may have been deprecated a long time ago, but I never, say, got any warnings from MT itself about deprecated behavior in a template. So after I upgraded and rebuilt, a lot of my content simply disappeared, because I didn't know about this change. And it's not even a matter of me not checking the release notes, as this issue was never mentioned in any of them as far as I can tell.
It would be nice if in the future there were some sort of warning log that's displayed after a page (re)build operation that tells you about behavior that's known to be changing or broken or the like, to make it easier to track these things down, and hopefully to prevent these nasty surprises from coming up during an upgrade.
Reported on Movable Type 5