After a tough week of battling a seriously impaired site, I wanted to document my error and solution since I didn't find any specific tips in the forums about this.
After installing several unused modules, I started getting this error message

Table 'database_name.node_field_instance' doesn't exist query: content_schema /* Anonymous : content_schema */ SELECT * FROM node_field_instance nfi LEFT JOIN node_field nf ON nf.field_name =nfi.field_name WHERE nf.active = 1 AND nfi.widget_active = 1 in /home/kbnet/public_html/sites/all/modules/cck/content.install on line 187

This resulted in content fields for several content types no longer being accessible (including embedded video, ckk email contact, and calendar dates).

Solution:

I compared the variable table of the drupal database with a backup and found that this row had been removed
   content_schema_version
I inserted it using phpmyadmin (old value was  i:6009; in my case) and that solved the problem.

Comments

Thanks so much for documenting this! Saved me hours of troubleshooting.

For anyone else with this error. The above fix is correct, and works well. However, if you have the content_schema_version variable exported in a feature, if you revert the feature you'll reproduce the error. Moral of the story: DO NOT export content_schema_version through strongarm.