The vast majority of illegal usages I have encountered are either postage stamps used as revenues on a document, or revenue stamps used as postage on postcards or covers.
Here’s one that’s a bit different, acquired laste weekend at CHICAGOPEX.
Illegal use of a revenue, not as postage, but rather as a postage due stamp. The 1-cent Franklin was cancelled on September 8, 1927. You can see the penciled ‘Due’ notation peeking out from underneath the revenue stamp. Then the additional stamp was affixed and cancelled one day later. The rate for post cards was 1 cent up until April 15, 1925, 2 cents from then until June 30, 1928, at which point it reverted to 1 cent until 1952. It is speculated that this stamp was inadvertantly used due to its similar appearance to the 1-cent postage due stamp.