ha_innodb.cc:

  Change error code to HA_ERR_ROW_IS_REFERENCED if we cannot DROP a parent table referenced by a FOREIGN KEY constraint; this error number is less misleading than the previous value HA_ERR_CANNOT_ADD_FOREIGN, but misleading still; we should introduce to 5.0 a proper MySQL error code

sql/ha_innodb.cc

@@ -285,7 +285,7 @@ convert_error_code_to_mysql(
 
         } else if (error == (int) DB_CANNOT_DROP_CONSTRAINT) {
 
-    		return(HA_ERR_CANNOT_ADD_FOREIGN); /* TODO: This is a bit
+    		return(HA_ERR_ROW_IS_REFERENCED); /* TODO: This is a bit
 						misleading, a new MySQL error
 						code should be introduced */
         } else if (error == (int) DB_COL_APPEARS_TWICE_IN_INDEX) {