Freedom FOREVER.

Stamps, like so many products that live digital lives, are susceptible to thievery. An unscrupulous yet correspondence-happy individual could ostensibly take an image of a postage stamp found online and use a print of that image to send birthday cards or holiday greetings or action figures sold on eBay. To prevent this, the U.S. Postal Service employs a crude-but-effective form of theft prevention: In the stamps it displays in its online store, the mail service simply crosses out the value of the stamp. 

So, for example, here's the USPS's Earthscapes collection, with each stamp's price -- these are Forever Stamps, which means they are literally priceless -- crossed out:

And here's the USPS's $0.32 Aloha stamp collection:

And here's the service's Santa-and-reindeer-themed Forever Stamp holiday collection:

Anyway, you get the idea -- postal watermarking, via slash mark. And I mention it because, as the blog One Foot Tsunami points out, this regular practice of digital safeguarding can lead to some unintended irony. Here, for example, is the USPS's Four Flags collection:

Yes. Ouch. (And also: touché?) Now, to be fair, there is literally nothing on any of the four Four Flags stamps that could be crossed out without occasioning some similarly ironic hilarity. And anyway, the watermarking is sort of -- just a little bit -- poetic. If not, one hopes, prophetic.

Hat tip Rob Meyer.