Patrick Dempsey does not make many career missteps, but I think this is a big one. He has signed on for Transformers 3. Never, ever, ever sign up for that third movie. Now, I am not counting James Bond movies in this discussion because they are their own separate entity and aren't based on an action figure. Ditto Harry Potter. You think that 3rd Twilight is going to be good? Did you see the first two?
Think to yourself of a franchise that has had a third part. The third part is almost always the worst installment. Does Sofia Coppola in Godfather 3 ring a bell? Police Academy 3? American Pie 3? Fast & The Furious 3? Home Alone 3? Jaws 3? Rocky 3? That one was actually not that bad. Usually actors are on the hook for the first two, but after that they can move on.
By the third installment, writers are different, the story ideas are gone and maybe a new team of writers is brought in. The producers figure there is a built in audience so they don't have to try very hard. And it shows. I have no idea what Patrick is doing. Maybe it is the huge paycheck or the chance to be in a potentially big summer movie for his kids. The movie will make $100M but it will be a sad remnant of the first one. As bad as the second one was, the third one will be worse. It always is.