Well, I’ll not tell a long story, how I debug, but come directly to the bug mentioned in the title. I tracked his existence down to BASIC 2.0 as used in the VIC-20, C64 and the early PET/CBM series and it seems, that it was never detected, documented or fixed.
It is related to temporary strings, the stack of descriptors for temporary strings, that has a size of 3, and the so called “garbage collection”, which in reality doesn’t collect garbage, but does a defragmentation of string storage.
Fixing an ancient bug like this must be a weirdly satisfying experience.
