Is this a prank or a murder case?

A detective is doing an investigation about a cupboard. Which the owner stated that he saw blood stains on the side of the cupboard.

Detective: hmm, why is there blood stains?

5 min after his thinking. He ask the owner.

Detective: Sir, do you know any reason or a clue behind this?

The Owner: I don't know. That's, why i called you detective.

The owner was desperately want to know, how his cupboard got blood stains at the side. 20 min after the detective research. The detective saw something and ask the owner.

Detective: Why is both of your arm bandaged?

The Owner: Ah, i was busy moving the cupboards when i'm cleaning and one of it slipped from my arms.

In the end the blood stain came from the owner. Thus everything is just an forgetfulness of the owner from what he was doing a week after.