From "Star Trek: The Next Generation: Firstborn (#7.21)"
[showing Lursa and B'Etor a knife
] This was dropped by one of the assassins. It bears the crest of your house.
Someone must have given it to the assassins to implicate us.
Counselor Deanna Troi
Why would someone do that?
In order to tarnish our good name.
You cannot tarnish a rusted blade.