I realize that the discussion of demonetized postage is presumably about those stamps used on cover after demonetization, but since the original topic of the thread is about improper uses, there is also the case of demonetized postage stamps used improperly as revenue stamps.
In my opinion, genuine uses of such are incredibly scarce. I've been focused on improper uses (both postage stamps used on document as revenues and the reverse, revenue stamps used on cover as postage, as well as the use of non-documentary revenues as documentaries) for almost 15 years now, but have only been able to acquire 4 examples... and it's been at least 7 years since I last saw any example available anywhere.
Scott # 14 strip of 3 used in 1867. Has a 2003 PF certificate. Purchased from the late Bill Langs on
ebay.

Scott #24, 2 examples paying tax on an 1868 receipt.

Scott #26, along with a 2-cent revenue, paying 5 cents tax on an 1864 affadavit.

Scott #26, on an 1869 estate receipt, with a 5-cent revenue stamp on the reverse paying the certificate tax for a court filing. This may be the only known example of an improper use of demonetized postage as revenue along with a legitimately paid revenue tax on the same document (albeit two separate transactions).
