(Shown above: NGC 50c 1964 kennedy half transitional on clad quarter planchet MS64)
Transitional off-metals are not something you would necessarily expected to find in circulation, but this lucky collector did, as detailed in the Coin World article shown below. A transitional off-metal is an error type involving a change in coin metals for a coin design, and occurs when a planchet made up of the old planchet metal is struck by dies intended for the new planchet material. An example would be a 1965 quarter (which should strike clad planchets) striking a 90% silver planchet (a planchet which stopped being used in 1964.)
Here are a few examples in our inventory of transitional off-metals:
We have heard of a number of collectors over the years finding rare transitional off-metals. This happens because of a few reasons, including that the planchets they are struck on are typically very close to normal weight, are the correct diameter, and so are able to escape the Mint's riddler's and quality controls. Additionally, they are often more or less the same visual appearance as a normal coin (for example, this 1977-D 40% silver Kennedy half is almost the same exact "color" as a normal 1977-D clad Kennedy--and so it is less likely to be noticed and pulled from circulation quickly.)
Additionally, it is an error that is likely to occur at some level whenever a new metal is used for a coin type, because there is a shift from the old to the new, and leftover planchets have to be all completely removed from the press and planchet bins, otherwise they will be fed into the press that is set up for striking the new metal type and a transitional off-metal will occur, and for the aforementioned reasons, will likely make it out of the Mint without getting caught.
The same is true of the 1983 transitional off-metal cents, which are 1983 cents struck on the bronze 3.1 gram planchet instead of the correct planchet, which is a copper coated zinc planchet weighing 2.5 grams.
Keep an eye out for transitional off-metals, because they are found from time to time.
Below is the article by Coin World's Paul Gilkes on the discovery of the 1977-D transitional.