Any syntax error in TargetExpression, CompletedExpression, or PendingExpression is returned as an error string as described in Error handling.

An error is returned if a race condition occurs during the update of multiple quotas using an expression. This can occur if another user updates the target, completed, or pending value between the web service reading the value from the database and writing the new calculated value. This is prevented by including the original target, completed, and pending values in the WHERE clause of the UPDATE statement. If zero rows are affected by the UPDATE, it is assumed the counts have been changed compared to when they were read. In the unlikely event of this occurring, an error is returned to the user who can refresh their view of the target, completed, and pending counts, and then re-run the update if necessary.