It's more common to be "wrong routed" but actually sign the route (but just not be going to that destination on that service) than to be wrong routed and not actually sign the route at all.
Though that's at my TOC where we do metro/suburban so sign lots of diversionary bits and bobs just rarely use them. I suppose a wrong route for a more "intercity" TOC would be potentially more likely to not even be signed?
But only the other day I was asked to take an empty train in to a depot I don't sign so I suppose it does happen!
