In the 10 Gigabit Ethernet MAC v13.0 (Rev. 1) and earlier, fragmented frames were incorrectly counted as undersized frames.
An undersized frame is one which is less than 64 bytes, but otherwise well formed, i.e. looks like a frame and the frame check sequence (FCS) is fine.
A fragment is a frame that is less than 64 bytes and where the FCS is invalid.