Letters and numbers
If you make serial numbers or passwords, don’t use 0 or o or 1 or l. Simply skip them as options.
If you want people to remember something, don’t mix letters and numbers together.
If you want people to be able to type in a code on a phone, don’t use caps.
The best passwords and serial numbers to share are actually a series of simple words. blueredrobin is way easier to type and remember than b2#3R4, even though they have similar security.
If this is so obvious, why is it rarely done?
Everything is designed, and design is marketing. It shows that you care, it makes the people you seek to serve happier, and it’s easier, too.