The possibility that a rare U.S. inverted Jenny (Scott C3a) was used to help pay postage for an absentee ballot in Florida in November's election got a lot of publicity when it was announced by Broward County elections officials, but it's almost certain the stamp is a counterfeit.
The story is told on the Web site of the American Philatelic Society, an international organization for stamp collectors.
The inverted Jenny is a 1918 U.S. air mail stamp. One pane of 100 stamps was printed with the airplane that is the central part of the design upside down. The stamp is in two colors, red and blue, and at the time it was printed, the paper had to be run through the press once for each color. The paper was inadvertently turned around, and the mistake was missed during the inspection process and sent to a post office. In theory, because the stamps were printed on a large sheet containing four panes, there had to be 300 other stamps with inverted centers, but only the one pane was found.
The full story of the stamp is told in the book "Jenny!" by George Amick, published in 1986.
Here are some excerpts from the APS report:

"On November 13, interested parties got a closer look at the controversial stamp in a warehouse in Fort Lauderdale. An image of the invert was transmitted to Mercer Bristow, Director of Expertizing at the American Philatelic Expertizing Service (APEX), at the American Philatelic Center headquarters of the APS in Bellefonte, Pennsylvania.
"It requires direct physical inspection of any stamp, including its paper and the printing method used, to pass judgment on its authenticity with absolute certainty. However, judging from the computer-scanned enlargement that was provided, Bristow concluded that the stamp was a fairly well executed forgery of the rare Jenny invert that has been known of for about the last decade.
"Bristow described the adhesive as 'a questionable item' based on five criteria. The transparency and thickness of the paper are not consistent with those of genuine stamps, nor does the blue color precisely match that of the originals. In addition, the number and therefore the spacing of the perforations along the vertical and horizontal margins do not tally with those of the 1918 air mail stamps, and do not even match one another along the left and right margins."

Assuming the analysis is correct, and there's no reason to question it, the good news is that there isn't a stamp collector somewhere trying to suppress the proverbial "urge to kill" a hapless spouse, employee or friend.