Continuity mistake: Mark Wahlberg cuts through the back seat to get access to medical supplies in the boot - other stuff is visible in there too. But shortly afterwards when he reverses it into the water, the boot pops open and it's now completely empty.

Shooter (2007)
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Directed by: Antoine Fuqua
Starring: Mark Wahlberg, Danny Glover, Rhona Mitra, Michael Pena
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Mr. Rate: Would've been a bad job to take, though.
Nick Memphis: How come?
Mr. Rate: Whoever took that shot's probably dead now. That's how conspiracy works. Them boys on the grassy knoll, they were dead within three hours. Buried in the damn desert. Unmarked graves out past Terlingua.
Nick Memphis: And you know this for a fact?
Mr. Rate: Still got the shovel.
Trivia: When Swagger is recuperating from the makeshift operation, it's shown that he has a third nipple below the left nipple.
Question: Why does Swagger include 5 tennis balls on the list of medical supplies he hands to Sarah?
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Answer: If you cut a slit into them and squeeze when the ball expands back it creates suction. So they could have been used to clear away any blood or pus when cleaning the wound before stitching.