Continuity mistake: At the end when the 3 Marine helicopters take off to rescue Burnett one of the 3 has their side door open. The position of this helicopter changes instantly between camera cuts. It goes from left to central of the 3. (01:26:50)
Continuity mistake: While speaking with the Admiral, Burnet's collar is turned up at the back of his neck. When he leaves and is talking to Stackhouse it's lowered and neat. When Stackhouse turns him round to speak to him, it's raised again. (00:11:40)
Continuity mistake: When the soldiers find Stackhouse one of them stands on his injury. When the shot changes to Burnett's point of view, the soldiers haven't reached Stackhouse completely yet. (00:28:50)
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.