I have used password-store (pass) to generate passwords and paste them to forms without realizing they were truncated and simultaneously those sites don't have the same maxlength on their login form.

Ah. On first read, I assumed you meant you had a webapp which was broken by these changes.

(This isn't the same person that answered you)

A particular case that I have seen is fields for account numbers that expect X characters, but people are copypasting the numbers from sources where they are represented with extra spaces for readability. (e.g. 1234 5678 instead of 12345678) And when they get to the validation, the last digit(s) have been lost.