i don't think so, if it's says there is X left, i add to my cart it should deduct and keep it in my cart so when i checkout it's fine. even if i checkout 10 mins later after adding it.
anyways no harm done i suppose, but this should be looked at. what i guessed what happened was i was on 3G then as my router came on my connection switched to wireless which "broke" or "changed" the session and my ( and i do hope it works this way ) reserved copy in the cart was unreserved.
if that is not the case then as i've said it should be looked at as this is making of trouble for anyone who adds to cart and it doesn't get reserved.
which brings me to my next question, if added to the cart it should be listed as such in my account and not just in a cart.
now i suppose the voucher is "used" for my account even if my item in my cart was suddenly out of stuck due to reasons unknown.