From the IDisposable Dispose method documentation:
If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times.
But what happens in you call Dispose twice on ReaderWriterLockSlim? A shiny ObjectDisposedException. Oops.
For the record I discovered this while unit testing the multiple dispose behaviour of one of my own classes not in production code. Annoyingly it makes testing the disposed behaviour of my class difficult as my tests will not be validating that my code is throwing the exception. I wonder how I report this…