The 1863/2 Three-Cent Silver shows a clear undertype of a 2 beneath the 3. This overdate is only available in Proof condition, as no circulation strikes are known. However, the reverse die shows a crack that also appears on some 1864 circulation strikes, indicating that this was a restrike issue, made after the Proofs of 1864, and before some of the regular strikes of 1864. This could only have occurred in a short window of time in late summer 1864.
According to Walter Breen (1988 Encyclopedia), Don Taxay discovered this variety in 1962.
This is a popular overdate in all grades, but especially so in Gem and Superb condition. The finest example is a single PCGS PR67CAM that would sell for a seemingly astronomical price if it ever hit the market (especially since a PR66CAM sold in 2011 for almost $50,000).
Stack's Bowers 8/2011:7238 - Eugene H. Gardner Collection - Heritage 6/2014:30123, $47,000