My fault! I don't think there is a Drupal error. I was trying to implement a new rule based onhttp://www.drupalcommerce.org/user-guide/shopping-carts-orders-and-line-... where I followed these instructions:
"To remove an item from the cart using rules, you must unset the price of the line item during "Calculating the sell price of a product." This is similar to the rule "Unset the price of disabled products in the cart" that is included with core. So you can refer to that as an example. Deleting a line item just involves setting the price to NULL (i.e. no value, not 0)."
I cloned that rule, but could not get it to work. During my various attempts, my cloned rule dissappeared (I don't believe I deleted it, but could have). Now, when ever I "add to cart" I get this error and can't get around it. The original rule looks intact, but I still clicked on revert just in case I screwed that up. That did not change anything. I have worked on this site (locally) for many days and is almost done. I am a novice at Drupal and don't have any SQL skills. Here is the complete error message:

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'commerce_unit_price_amount' cannot be null: INSERT INTO {field_data_commerce_unit_price} (entity_type, entity_id, revision_id, bundle, delta, language, commerce_unit_price_amount, commerce_unit_price_currency_code, commerce_unit_price_data) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8); Array ( [:db_insert_placeholder_0] => commerce_line_item [:db_insert_placeholder_1] => 68 [:db_insert_placeholder_2] => 68 [:db_insert_placeholder_3] => product [:db_insert_placeholder_4] => 0 [:db_insert_placeholder_5] => und [:db_insert_placeholder_6] => [:db_insert_placeholder_7] => USD [:db_insert_placeholder_8] => a:1:{s:10:"components";a:1:{i:0;a:3:{s:4:"name";s:10:"base_price";s:5:"price";a:3:{s:6:"amount";s:3:"500";s:13:"currency_code";s:3:"USD";s:4:"data";a:1:{s:10:"components";a:0:{}}}s:8:"included";b:1;}}} ) in field_sql_storage_field_storage_write() (line 448 of C:\Users\Rich\Documents\My Web Sites\Drupal Commerce Kickstart\modules\field\modules\field_sql_storage\field_sql_storage.module).