The 1942-D over Horizontal D Jefferson Nickel is a result of a Mint employee punching the mint mark in the wrong position on the working die. Up until late in 1989 the Mint employee would hand punch the mint mark on to the working die and many times the mint mark would accidentally be struck the wrong way.
This variety is relatively strong and visible with the naked eye. It is also somewhat popular with Jefferson Nickel collectors. It is considered quite scarce with possibly less than a few hundred examples known. In Uncirculated grade it becomes very scarce with probably less than 100 examples known.