While writing tests it can be easy to accidentally change any global state that exists in your application. I've previously written about Redefining State Within a Test; however, redef-state and with-redefs only help you if you redefine all of the affected state. The situation is even more problematic due to the fact that accidental state alteration often doesn't cause issues until a completely unrelated test suddenly fails. After being bitten by this issue a few times, I added (to expectations) the ability to warn when global state is modified by a test.
As of version 1.4.24 if you add (expectations/warn-on-iref-updates) anywhere, then expectations will provide you with a warning whenever any global state is altered.
While you can add that snippet anywhere, I prefer to add it to the expectations Before Run Hook. There's an example expectations_options.clj in the expectations codebase that shows all of the code you need to enable this feature. Simply add this file or add the function to your existing file and you should see a warning on any global state alteration. If you're not sure where to put this file, refer to the Before Run Hook blog post.
Here's an example warning (generated by running the expectations tests).
WARNING: success.success-examples:280 modified #'success.success-examples-src/an-atom from "atom" to "another atom"
The warning should let you know which test is doing an unexpected modification, and the to and from values should give you an idea of where in the source the alteration is occurring.