- Chip with less memory or lower clock different from original STM32 F405
- Prototype chip (ARM A, ARM N, ARM 2) with bugs instead of production latter silicon revisions (ARM Z, 1, 2, Y)
- Shorts or not enough solder paste
- Bad capacitors causing resets when connecting board over USB
- Bad regulators
- Missing ESD protection diodes on Flexi, Flexi-io or Main port (except Nano which has none by design).