Just after Ace has been hit with three darts he comes to rest against a tree and tells Spike "I think we lost them" and is then hit with more darts in his back. The video shows four darts going in and the audio sounds like four. About a second later we see his back in profile and there are five darts visible. Not to mention the painfully obvious target block under his shirt.
I am hoping that different V.C.R. brands keep near the same time (i use a Panasonic) to see this slip up fast forward to 00:52:38 This should bring you to right before he says "I think I lost them" Be sure to pause right has they do the side profile to get a good view. I also have to agree there is obviously a board under his shirt :)