So you know I done that on my computer in Picasa3 & Photobucket as a gag. It's not an actual stamp just a text graphic I overlayed on an image of my Molly Pitcher.
Back to the topic, the overprints were often refused as valid postage by unaware postal workers. The public complained, and once the existing supply was used, the trial program was stopped.
Thank you kacyds I was always curious about why they used overprints and also why (the US anyway) stopped using them.