My understanding is that runts are fragments of packet collisions. Runts are anything under 64 bytes frame. If collision get detected after 512 bits, it is marked as late collision.
Followings are my assumptions. Please corrects them if wrong :)
Runts get 100% detected and dropped by fragment-free or store-and-fowarding switches. Cut-through switches could detect and drop the runt if the destination mac address in the runt is corrupted (In this case, the frame is less than 6 bytes). Otherwise, cut-through switches won't be able to detect it since it only checks the destination mac address. If cut-through switch doesn't detect the runt (6B <= size of runt < 64B), it will be dropped at the end-device(PCS).
If store-and-fowarding switch detects the late collision and drops the frame, the sender will retransmit after retransmit timer (BTW, who decides and how to calculate the retransmit timer?).
