PICO-8 will gladly accept export("sfx_%x_%d.wav") because the string argument contains %d, but the format string is not sanitised and the SFX index will be caught by %x, and %d will then pick who knows what memory location.

Funny things can also happen with %s, %*s etc.; export("%s%s%s%s%s%s%s%s%s%s%s%s%d") is almost a guaranteed crash.