Some customers report that they can't log in on the frontend, even with a correct password. I've seen it happen sometimes, the page just reloads with no error messages. Deleting the cookies solves the problem, but as you can guess I can't ask to do that to everyone.
My current Session Cookie Management settings are:
Cookie lifetime: 86400 Cookie path: / Cookie domain: .domain.com Use HTTP Only: Yes Cookie Restriction Mode: No