I was doing the Dark Brotherhood quest where you kill Valen Dreth in the prison, and I made it to his cell without being seen by the guard. I open his cell reveal myself, and talk to Valen. I manage to get in a few hits and he runs while the guard starts attacking me.

Here's the good part. I yield and the guard accepts, then finishes Dreth off for me! I left with no bounty out the front door lol.