1970 New Zealand (overprinted 1971 or 1973)
This seemingly simple stamp from my shoebox led me down a bit of a rabbit hole. I picked it out because of its bright colors and appealing design of a Magpie Moth and I figured the price change didn't get in the way too much. So the original stamp comes from this lovely set of pictorials from 1970 and was designed by Eileen Mayo, but like all printed matter that goes through multiple passes, errors can occur, which you can see documented here. When the rate increased in 1971, there was no longer a use for 2½¢ stamps so they decided to overprint the stamp with a surcharge (which still kind of blows my mind as it seems like a lot of extra, and not so attractive, trouble to go through, but there were more than 37 million stamps left), and apparently the way the surcharge was printed can affect its value. There are actually three variations of the overprint: photogravure on un-perforated sheets and letterpress on perforated single sheets done in England in 1971, and then a letterpress overprinting in New Zealand in 1973. It's kind of amazing to think of all the details and work that went into this little stamp.