ResteasyProviderFactory has a public static getInstance() method which returns a singleton instance. Many tests use that method to get the ResteasyProviderFactory and then customize it with providers or such for the specific test scenario, without cleaning the singleton afterwards.
This is a leak and potentially leads to unexpected behaviors / intermittent failures in the following tests.

Note, on master the ResteasyProviderFactory has static newInstance() method that creates a new instance of the factory.