STM32 gotchas
206.In 'G0Bx, erratum makes both FLASH banks barely usable for code

As in many STM32 models representing the higher-end of a given family, STM32G0Bx/'G0Cx has its relatively large FLASH (up to 512kB) split into two banks (except in models marked to have only 128kB of FLASH).

The Prefetch failure when branching across flash memory banks erratum, pertinent to both rev.A and rev.Z of the chip, effectively restricts the two banks' usage to:

User berendi put up a couple of relevant questions, answers to which may make both banks usable for code with certain cludges (such as trampolines in RAM), but ST did not comment.