Back in the day, retail software was shipped on physical CDs that often had a printed "CD key" label affixed. The (usually alphanumeric) key on the label was unique to that particular copy of the product, and it needed to be entered during setup for the software to work. The keys had a special property that allowed the installer to determine if the key was valid without contacting a central server of any kind, while still being generally impossible to mash random letters on the keyboard and produce a valid key.
Without giving specifically identifiable examples, there are a number of legacy software products and games that I've seen folks discussing installing in recent months, often with missing/lost keys, and common advice is to use keys that look like
buttheadbutthead during the key entry dialog in the original, pristine installer -- and somehow these keys generally work. Compared to real keys (which looked random, like
BWHP-PWJU-VTV1-94M5) these keys are obviously weak and, perhaps most importantly, very easy to memorize.
It got me wondering why such keys are accepted in these unmodified retail versions. Is it that these patterns were intentionally installed as "backdoors" by the developers to simplify the development/QA/support process? Or is it more likely that the key validation used some form of home-grown hash/checksum procedure that had blatant but undetected weaknesses that end-users later found and exploited?