Thanks for reporting this! I think it's actually an obscure and hard to track down bug in PICO-8, since I've seen a few different versions of this message reported, all happening in seemingly random places. As such, I'll try to raise the issue with PICO-8's creator, but unfortunately it's not a guarantee that we'll be able to reproduce it (let alone fix it). The one good thing about it is that it only happens very rarely.