What causes DatabaseSchemaObjectExistsException error on existing table while reverting feature?
This is usually caused when the initial feature revert or install went bad. The feature has attempted to created a field (I've only seen it happen with fields) and it's got as far as creating the base table and it's revision table but as far as features is concerned the field doesn't exist. Features then tries to re-create the field but Drupal errors out with that error message because the fields tables already exist.

My usual solution to this is to take a backup of the fields tables (including revisions) and then drop the effected tabled. You should then be able to revert the feature successfully and restore the data from your backup.