I've got three Feliway plug-in diffusers which have been in a cupboard for a few years.
Since Kit got stressed by our furniture move I've pressed them into service.
I kept smelling something 'odd' and eventually tracked it down to one of the diffusers which was too hot to touch, and the plastic was brown in places.
I wonder if it was a fire waiting to happen?
I'm definitely keeping an eye on the other two.
They do seem to be working to calm the cat down, but raised my blood pressure for a while.