Today I got a "sale" of one of my books that was free in Spain. Problem is, that book isn't free anywhere now and hasn't been for months. It's also listed on Amazon in Italy and Mexico as free. I've already checked my other titles, which are okay, and emailed Author Central asking Amazon to fix this. My pricing on my Bookshelf is correct and doesn't show free anywhere.
(Recently, Amazon suddenly had wrong prices for many international stores--but those were listed incorrectly in the Bookshelf and I checked them all and fixed them all.)
This is the only book I have ever set to free even for a brief period of time. My belief is that Amazon made this error because somewhere in its computers that title's history as free still exists--and the computer twitched in a weird way, so suddenly that old price appeared on random Amazon stores.
I guess the safe stance is to never do free, or conversely, do permafree, and no waffling back and forth over a couple of years.
It's not a title that sells well, so I'm not out any serious money. I did ask Amazon to pay me my royalty for the "free" book that someone bought but I'm not holding my breath over that.