It took me a while to figure out what was going on, but whenever my form details change (or the login form changes), when logging in, 1Password asks if I'd like to save the details - as the entry already exists, I choose to "update existing". Everything appears to work normally, until I attempt to login again, and "suddenly" I can't login. I looked at the entry in the 1Password app, and notice that the password has changed!
Likely unrelated, but this has happened mostly on my Bank Account login, which is most irritating. I now have about 15 "previously used" passwords. It's just lucky that I remember the first couple of characters from that password to pick it from the list!
1Password Version: 1Password 5 Version 5.3.2 (532001) Agile Web Store
Extension Version: 4.4.1
OS Version: OS X 10.10.4
Sync Type: Dropbox