I've had this happen on pure length. I believe KeePass defaults to 20 characters. I've seen websites accept 20 characters on sign up, but internally, the log in only accepts 12 characters, but it doesn't truncate the input either. I had to enter the first 12 characters and submit the form, and it worked.

I was completely baffled on why it was designed that way - if you're going to truncate the password, the login field should do the same.

Omg you must have incredible luck when filling out sign in forms. There must be some sort of sadistic instinct on the types of people who design password forms. I’ve had passwords rejected for being too long (over 15 characters), including the “wrong” kind of special characters, having the same character repeated twice in a row, not having enough numbers, just to name ones I can remember off the top of my head. Oh the best ones don’t tell you the rules until after you’ve been rejected.

A special place in hell is reserved for those websites that consider themselves too cool for a password manager. They actively block auto fill or cut & paste in the password field. I don’t envy the 1password devs for having to put up and work around this stuff.

> I've never had a website outright reject certain special characters,

This is exceedingly common for US Banks. You'll find, usually only after pasting in the newly generated random password and clicking submit, that the "your password must include at least one number and two special characters" description up front failed to also include: "oh, also, we do not allow use of the character % in your password" (or some other character).