Look for the following...
1) Two feathers varieties of many dates, there are over 30 with the small feather missing. They are all collectible and desirable.
2) Three and one half legged buffaloes, particularly 1913 type one, 1916-d, 1917-d, 1926-d, 1927-d, 1936-d, and of course the (in)famous 1937-d three legger.
3) Doubled dies of 1915 (obverse), 1916 (obverse) 1917 (two different reverses), 1918 (reverse), 1930 (mulitple obverse and reverse doubled dies), 1935 (reverse) 1936 (obverse and reverse).
4) Overdates: 1914/3, 1914/3-d, 1914/3-s, 1916/1916, 1918/7-d.