For this site, the 51st CB field out of 76 total fields had been the "phone" field, which was a user-defined text field. After upgrading to CB 2.1.0, this field was replaced by the "alias" field, which is a "Predefined name and username" field. The title of the field is the same as what had been chosen for "phone", but all other attributes are the "alias" field, where "alias" is apparently the URL for the profile. We didn't have an "alias" field before this.
If the "alias" field was to be added as a new system field, I'm assuming it should have become the 77th field. But it acted as if it thought there were only 50 fields used.
Luckily, I did this on a test site. But I was hoping after many changes to move the test site to production. Is there any simple way to recover from this, and is it the bug it appears to be?