When does -1 not equal -1? When -1 started out as the string "-1", of course!
Please examine this source code closely, then run it for a surprise fixed point math bug or sommat.
(If you are visiting this thread in the future, after zep has updated the web player, refer to the cartridge label for the original output.)
(I do not know if this is a duplicate symptom of another bug that has already been fixed in 0.1.11, sorry...)
Well, at least in the process of making this test cart I stumbled upon the workaround for it. flr("-1") is fine.