Weak bits
A copy-protection trick where bits were deliberately written with unstable magnetisation, so they read differently on each pass — impossible to copy exactly.
Also called fuzzy bits. A protected floppy deliberately wrote a stretch of flux with no clean transitions, so the drive's data separator returned random values there on every read. The original program checked that the region read inconsistently from pass to pass; a bit-for-bit copier, which reads once and writes a fixed pattern, produced a region that read the same every time — and failed the check. It was one of many disk-based tricks of the 1980s (alongside extra tracks, deliberate bad sectors and non-standard sector sizes) that made games hard to duplicate without a hardware copier. That self-clocking read path is exactly what they exploit.