The Apple II's disk controller could easily write "flaky bits" in software. By telling the controller to write three or more consecutive zero bits, you'd produce a sector that read back unpredictably. So your copy protection scheme would be to turn off sector checksums and then read your test sector a few times in a row. If the sector came back identical each time, then you were likely a copy rather than an original.

After reverse-engineering the verification code in a few different games, I wondered how the publishers produced those weird sectors. I called them "weak bits," coincidentally, because my theory at the time was that they modded the disk head to write the bit weakly so that it couldn't distinguish a one from a zero during readback. A friend at school had a copy of Don Worth's Beneath Apple DOS, which absolutely blew my teenage mind. Until reading that book, I didn't think that any single human could understand and clearly explain a complex system so thoroughly.