OP: yes, some users ARE that stupid. But we can't kill 'em, so we're stuck with them.
I've found that dumb users can't figure out activation email links or password resets. For activation links being sent to their email, most didn't add the website domain to their safe sender list and it would just get bounced to the junk folder, or the user wouldn't realize they had to click the activation link- they assumed that just by receiving the activation email everything was all set. Some users got pissed that they got any email from the website and flagged the activation email as spam.
Some don't remember they were asked to pick a username and try logging in with their email address instead and wonder why the login won't work.
Requiring secure passwords was also a no-go for the community site I was hired to work on. The users couldn't remember anything and they had a 99% failure rate typing their passwords in- even after they wrote the password down somewhere so they wouldn't forget. They would SWEAR it didn't work for them and they'd get insulted when I suggested a copy/paste of their password- meanwhile I'd already checked their passwords and they magically worked for me...
We wound up changing the login form so that people could log in with either their username or the password registered to their account (provided they got the password right).
We also changed the activation link to read "click this link to finish creating your account" instead of showing the link to the activation page.
I think the worst of all was one user who wrote me a SCATHING email about her user account profile being made public for all to see. She didn't realize (and refused to be told) that she was logged in looking at her own profile settings page. No matter what we told her, she just freaked out and wasn't open to learning about how user accounts on community websites worked.
Since I was the only person who knew how to use the site administrative tools I was the one stuck with helping the dumb-dumbs. Then again the community site I was hired to work on didn't appeal to the sharpest knives in the drawer if you know that I mean... That was part of the problem.