It's amazing how few stamps get canceled properly - I'd say about one in 20 have no markings to indicate the stamp is canceled.
I don't see how they're able to distinguish stamps removed from letters that have been through the mail, from stamps that are mistakenly placed on envelopes and peeled off again (I presume, a perfectly legal act).
As it happens, though, I think the postal workers really couldn't care less. As part of a bet with someone, I posted myself an anonymous letter which was "stamped" with a very badly photocopied, very badly cut out, black and white grainy 1st-class stamp. To my amazement it arrived with no problems whatsoever (it did have some stranger than usual markings where they stamp over the stamp - maybe it was done manually or some such). I was actually rather disappointed.
