Audio problem: When Burnett is running over the bridge yelling for the car his mouth doesn't move. (01:02:50)
Audio problem: At the start a yellow jacket says to Burnett about loosing a finger. However, his mouth doesn't match what he is saying. (00:04:40)
Answer: The two seats collided in midair before their chutes opened and Stackhouse yelled, "ow!" Not realistic because in the 5 seconds it took for the second pilot to eject they would have been far apart.
But you see his leg get injured before his leaves the jet. I think the original question refers to how did that happen.
Ssiscool ★
The injury to Stackhouse's leg was from a pen attached to his knee pad. When watched in slow motion, you see it disintegrate and somehow throw shrapnel into his leg. It makes little sense, I believe it was merely a plot point to make Stackhouse unable to travel out of harm's way. This prompted Burnett to leave him unattended for the bad guys to find him as he went to higher ground to get better reception on the PRC-90.