Eddy Merckx
The blackest day in Merckx's career was in 1969, when he crashed in a derny race towards the end of the season. A pacer and a cyclist fell in front of Merckx's pacer, Fernand Wambst.  Wambst and Merckx crashed. Wambst was killed instantly, and Merckx suffered concussion and fell unconscious. This accident cracked a vertebra and twisted his pelvis. Afterwards he said his riding was never the same, because he always be in pain, especially while climbing.