The cast of "Transformers 3" has been shooting like crazy in Chicago -- but this weekend things got a lot more interesting ... when Patrick Dempsey popped up on set!

When Dempsey signed on to the flick, he was set to play "Megan Fox's boss" in a role "significant to the plot" -- but since Fox is out, we're guessing he'll be boss to newcomer Rosie Huntington-Whiteley instead.
(See Josh Duhamel, Tyrese on set!)

Check out Patrick, Shia LaBeouf and all the gun-toting action!

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