It isn't exactly unusual for an actor to slap another actor for real when a scene calls for it, but usually such things are discussed first, if only so the other actor knows what to expect when the director calls "Action!"
Not so in Dustin Hoffman's book though. He slapped Meryl Streep for real without telling her he was going to do so during the filing of 1979 divorce drama Kramer vs. Kramer.
"I think [Hoffman is] very, very gifted, [but] it wasn't the most fun I've ever had on film," she later said of the incident.
Sure, one could argue that Meryl's reaction to her co-star's surprise attack brought way more authenticity to the scene, but on the other hand Dustin Hoffman just outright slapped Meryl Streep in the face. And that's just not on, is it?