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wow....maybe the machine just automatically cancelled it without checking if its a stamp
This is a problem that USPS has pretty much given up on after making early efforts to combat.
The automatic sorter/cancellers are basically only checking for one thing in regards to a stamp -- whether there is luminescent tagging present. The machine optics are not looking for a denomination or a shape or a design. So anything that luminesces at the proper wavelength range for the machine optics will pass. Yeah, I've seen luminscent stickers that passed through the system, and even hand-drawn illustrations on luminescent selvage. I don't condone doing those things, of course.
This is why USPS no longer tags low-denomination stamps (i.e., 1c, 2c, 3,...). To keep unethical people who know, from using lower denomination stamps for first class postage. Of course, there's nothing to keep them from using older tagged 1c, 2c, 3c... stamps, except ethics.
For the self-adhesive label parts, USPS has tried circleX die cuts to prevent intact removal of labels, "NOT VALID POSTAGE" messages... but some people still try (sometimes unintentionally).
The only resource that USPS has to stop this problem is an observant letter carrier. But with stamps with no denominations and stamps that look like stickers now, even the letter carriers can't figure out what's a real stamp and what is fake.
Penner1 has an interesting cover to have. What I like about it is that it was done unintentionally. A sort of "natural" cover, rather than some philatelically contrived cover.