That stamps are issued with the name of the issuing country and the value is well known. Also that Britain didn’t place its name on stamps, just the image of its head of state. And the value of a stamp in many countries is now shown with a letter or number. But stamps with overprints, to change the original stamp intention, are not seen so much nowadays.
But in former years they were quite common, and the collection of overprinted stamps was a hobby practised by many. Stamps with high print runs were often overprinted and used when circumstances changed in a country. For example, Greece issued a stamp in 1911 with a picture of Hermes, the messenger of the Greek gods, to the value of 1 Lepton. Two printing techniques were used to produce the stamp: intaglio and lithography. There were also recognisable colour differences, from light green to dark green.
In October 1912, the stamp was overprinted: ΕΛΛΗΝΙΚΗ ΔΙΟΙΚΗΣΙΣ (Greek Administration) and used in the Greek-occupied part of Turkey during the First Balkan War. The overprint was applied from bottom to top and also from top to bottom in black, light red and dark red ink.
The print is not always placed precisely in the middle, sometimes towards the left or right, or up or down. In the last case above, at the top side of the stamp you can see some letters.
Some overprints displayed damaged characters, such as the letter Η in the first line and the letter Δ in the second line shown here. There are also differences in the thickness of the letters (light and bold) as can be clearly seen in this block. Sometimes an overprints might have a missing letter.
On November 1, 1916, the same 1 Lepton stamp appeared imprinted in red with a symbol in the shape of a crown and the letters ET.
This overprint had nothing to do with a visitor from outer space but represents Ellinikon Tachydromeion. This means, as far as I can translate, Greek postal or courier service. This is just a small example of the possibilities offered by Greek stamps with overprints. Look in the Greek stamp catalogue, the Vlastos/Karamitsos, or a general catalogue such as Michel!