That isn't a built in error message that comes with FileMaker as far as I can tell. Something in your database design is responsible for displaying it. You'll need to take a closer look at the design of your layout and maybe the definition of your category field--a relationship could even be involved to see why you are getting this text.
A relookup solved the problem, but I stil don't know what caused it in such a sporadic pattern.
Relook up copies data from the related table into the field from a related table for every field with a looked up value field option that uses the selected field as a matchfield in the relationshp specified in the looked up value setting.
I would guess that the "if no exact match, Use:" setting was set up to return "Can't find". You'll need to investigate what event or events is triggering a lookup from that related table at a time when the value in the match field in the parent record does not match to any record in the child (Lookup) table.