How not to do repeated fields in web forms
There's a certain sort of web form which really wants to make sure that you've entered something correctly, so they ask you to enter it twice in two different fields. You've probably seen this in some web form sooner or later; this is the 'please enter your password again in this field too' or 'please re-enter your email address' field. I tend to think that this is bad on its own, but I've now seen an even worse implementation of this basic idea, which I'll call an anti-confirmation field, one that's practically designed to create errors.
(My web password for this site was a strong random password, as usual. Random jumbles are hard to transcribe accurately by hand, especially when they jump back and forth between character case.)
I suspect that the website designers justified this by saying that they were worried about people entering a bad email address by hand in the first field and then 'confirming' it by just cutting & pasting it into the second field. However, even at its best this logic doesn't work for password fields since browsers don't let you copy the plaintext content of a password field once you've entered it. I also suspect that the designers do not have any actual data on how many genuine errors this prevents (versus how many artificial errors are created).
Sidebar: how to measure the numbers
Assuming that you've committed yourself to (anti-)confirmation fields in the first place, you just need to track field values across time when a submission fails because of mismatched fields. In a transcription error the first of the two fields will turn out to be correct (ie, the same as the final submitted value) and the second field will change. In a genuine error the first field will be different between the failed submission and a subsequent valid one.
Doing this with email addresses raises basically no security issues. If you do this with the password field you'll want to one-way hash them somehow in your tracking data.