I forget my passwords a lot to specific accounts, so I use a few variations of about 4 passwords whenever I make an account somewhere. Basically, if I forget my login I just guess the variations of those passwords, and one always works. If those websites kept those wrong passwords stored, then they could basically log in as me anywhere. Scary to think of, eh?
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You should consider using a password manager. Or, as an alternative, you could try sticky notes and leave them in a drawer if you aren't worried about someone breaking into your home. Please don't reuse passwords. Thieves already have plenty of other ways to steal your personal information. No need to make it easier, yeah?