It is likely that the driver (and/or Mongoid) leak sessions in certain cases.

List of possible session leaks:
I believe that the driver leaks a session when the Mongo::Collection::View#each method is called (via any Iterable method) without a block and then an error occurs.
The driver also leaks a session when methods that take a block (like any?) are called without a block argument.
The driver might leak sessions during transactions

We should use session tracking in our tests to detect session leaks and address them.