Florio cites Judd Zulgad's notes, the best account of the bizarre injury:
The odd thing about what happened to Harvin on Saturday is no one saw him suffer an obvious injury. He did make what was perhaps the catch of the day in 7-on-7 drills when he bobbled a pass from Tarvaris Jackson but managed to catch the ball despite tight coverage from cornerback Asher Allen. Harvin's helmet popped off on the play as he made the catch, fell to the ground and then got up and tried to run.
Offensive coordinator Darrell Bevell said in his post-practice news conference that he didn't even know Harvin had left on a cart until told by a member of the media relations staff.