captcha fails when uid=0 not first row in user table

I have a site where the anonymous user, uid=0, was not the first row in the user table. Took me hours to figure out that this was why captcha would not work validate the input for the math problem. Once I exported the database and reinserted it with the uid=0 record first, captcha worked properly.

This is a fairly old Drupal site (probably about five years). Not sure how uid=0 ended up being later than the first record. Perhaps others having trouble getting captcha to work are having a similar problem.