The 1905-O Micro O Barber Dime was created when a die sinker used a mintmark punch intended for a quarter dollar which, ironically, is smaller than that used on the dimes. According to Walter Breen, this variety was discovered by Howard Newcomb circa 1912; yes, that's the same Newcomb who is famous for his reference work on die varieties of 1816-1857 large cents.
This is a relatively scarce variety made all the more popular because it is the only major variety of the series. The finest example is a single PCGS MS66.
Quickfinder Notes: The Micro O mint mark is only about half the size of the normal New Orleans issue. This is ironic, as numismatic expert Ron Guth points out above, because this undersize fellow was used on Barber quarters! The reason for this use was the very constricted space between the eagle's tail and the R of QUARTER. The Micro O mint mark is ROUND in shape with very thick side walls and an OVAL center. It's position is EQUIDISTANT and AWAY from the ribbon knot and denticles. The normal size mint mark is OVAL in shape with an OBLONG center. It is placed VERY CLOSE to the ribbon knot.