Revert to old password
Use Case:
I would like to change the password for an account. I generate a new password that is immediately stored in Proton Pass. If the password change failes I can not access the account with the old password again since this is overwritten by the new password.
Reverting back to the old password would be really beneficial, since it will save a lot of time.

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Ken commented
This duplicates the idea with more votes for remembering old passwords when they're replaced
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Roy commented
This occurs in the password managers that I have used (LastPass, 1Password, Proton Pass). Each website seems to be different in password requirements and sometimes the site disallows a password, while the password manager has already stored the new password. Not sure if its possible or technically the right thing to do but only save after the password has been accepted.