My favorite is when systems will stealth truncate your password without telling you, but only when setting it. For some reason I often encounter this with systems truncating to 20 characters.
Set 24 character password: no error (secretly truncated to 20 characters).
Try to log in: credentials invalid (it checks the full 24 character one against the 20 character one).
Go to reset to what it should be, password can’t be the same (again, stealth truncating to 20 characters).
My favorite is when systems will stealth truncate your password without telling you, but only when setting it. For some reason I often encounter this with systems truncating to 20 characters.