Your OTP love to prank each other.
One day, Person A comes home to find Person B lying in their kitchen with a knife stabbed through their chest.
They laugh it off, go over to pull the fake knife from B's 'corpse'.
Up to you on how they react when they see that the blade is made of metal and they can't find B's pulse.  

Bonus: They realized that they were out drinking last night and finds out that they (A) stabbed B after a heated argument while drunk.