A while ago, I put a stamp on an envelope, wrote the address, and then didn't need to mail it. I ripped open the envelope to get my document out. I saved the corner with the stamp, thinking I could use the stamp on some other package.
Sure enough, I decided to mail some CDs in a box. I tried to peel the stamp from the torn envelope. No luck. So, I neatly snipped around the stamp. Then I made a loop of sticky tape and smashed the stamp, with envelope paper backing, onto the box. All the other stamps I used on the box were new.
I weighed the box. 13.9 ounces. That meant I couldn't drop it into a mail box, but would have to hand deliver it to a post office. I biked to a post office. I waited in line. The cashier told me that I couldn't mail the package with my pre-stuck stamp. She cut through my sticky tape, handed it back to me and made me buy a new stamp to put on the box.
I asked her how I was supposed to use the stamp, which I'd paid for. She had no idea.
I suspect the pre-stuck state was obvious because the box was brown. Next time, I'm going to stick it to a white envelope that weighs less than 13 ounces and drop it in a mail box. Maybe it will get scanned by a machine and nobody will care.
But I'm posting this as a warning -- the a stamp is not a stamp, not worth the money you paid for it, if you change your mind about where you want to stick it.