Properly storing password (Hashing and Salting)
Passwords should never have been stored as plain text, but they were and when databases were compromised there was no protection. Then again people shouldn't reused passwords but they do, all the time. However with services like LastPass, 1Password and iCloud keychain hopefully people will start to shift away from reusing simple passwords. Regardless we as developers should err on the side of caution and always ensure that when we store passwords it's in a way that prevents anyone who would happen to get them to use them maliciously.