One transgression that often begs forgiveness on set is forgetfulness. The job of an actor is stressful and mentally taxing, and every so often, someone forgets their lines. Typically, though, when this happens, the director will call, “Cut!” At the very least, we don't expect these incomplete executions to make it into the actual film. The actors are supposed to refresh their lines and try again. But sometimes, forgetting one's dialogue is the most true-to-character thing an actor can do.
Of course, this is only true if you're lucky enough to play a character who's, well, a bit of an airhead. Derek Zoolander in the 2001 Zoolander film is probably the poster child for this kind of role (and we know how much he loves to see his face on a poster). In the scene in question, Ben Stiller forgets one of his lines, so he simply repeats something he said just a few moments earlier.
“But why male models?” he asks, wondering why male models are chosen to become assassins. David Duchovny's character offers him an extensive explanation involving male models' advantageous physical condition and their ability to penetrate exclusive circles, not to mention their most important quality — the fact that they don't think for themselves. A few lines later, that proves to be true, as Zoolander re-asks his earlier question: “But why male models?” Though it's actually a result of Ben Stiller forgetting his line, the ditzy moment reinforces Zoolander's character.