Deliberate mistake: The French NATO rescue team is flying an old, Russian-made Mi-17 helicopter which is not in service in France. It was probably rented in Slovakia where most of the film was shot. The Mi-17 is easily distinguished from the French multi-purpose helicopter Puma: the Mi-17's landing gear is supported by huge rods, its side windows are round, and it has a sleek boom with the tail rotor on the left side. The Puma has smaller gear support, rectangular side windows, and a thicker boom with the tail rotor on the right side. (01:15:42)
Deliberate mistake: The aft cockpit of the F/A-18F is not simply a 'Navigator' as they say several times in the movie. It is the Weapons Systems Operator, or WSO (Wizzo).
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.