Some of my users can not login

I have over ten thousand registered users on my website and a handful of those people can’t login. I’ve tried various things to duplicate their situation like disabling java script and cookies, but I can’t duplicate their login error which states:

Error

There are two reasons this error may have occurred:

You did not click on the link in the confirmation email that was sent to you. 
Check to make sure you didn't incorrectly type your email address or password.

Of course, the users always very carefully types their password and confirm their accounts. Can anyone help me with suggestions on why this affects so few out of over ten thousand people? What am I not seeing?

Thanks!

If you worked on a Help Desk (or within earshot of one) you’d realise that the Rule 1 of the basic rules of human stupidity suggests you have vastly underestimated the number of people affected.

How can you know that those few have actually not had Caps Lock on as they re-typed their passwords?

Not to state the obvious, but have those users actually clicked on the activation link in the email sent out to them?

Yes, all of these problem accounts have confirmed their account. I can always login just fine when I retrieve their username and password from the database. It’s very perplexing.

Maybe they have cookies off, or a weird browser?

If you are convinced their local setup is to blame then add a logger to collect the duff info, browser capabilities, associated user name and password etc.

Sqlite is ideal for these little jobs.

That may prove/disprove the user/pass issues and may do the same for browser set up.

I’d also consider removing any over-reliance on JS for the form to work as expected (especially, give the form a simple html submit button).