My clients' experience is that you could easily have a problem if you leave after the expiration of your visa; other clients have had to pay a fine (even when circumstances were completely beyond the traveler's control). This happened as recently as July of this year out of Moscow.

We generally circumvent this problem by issuing the invitation for the entire 30 day period (hotels and most tour companies will only give you an invitation for the period of your reservation).

Marc David Miller