docs: added note on hanging servers from testing

docs/next/en-US/fundamentals/testing.md

@@ -60,3 +60,15 @@ It is entirely permissible, and indeed encouraged, for apps that have more advan
 Common uses of custom checkpoints are particularly when combined with the [suspended state] system, if some of your pages fetch state on the client-side, and you want to test for that all working correctly.
 
 If a checkpoint is not emitted, tests waiting for it will fail with a timeout error.
+
+## Hanging servers
+
+Note that *some* critical errors during testing will lead to process termination in such a way that Perseus sometimes won't catch the child process failure correctly, which may lead to a server being left open on your computer. These are easy to terminate manually though: on Linux you might do this:
+
+```
+netstat -lnp | grep 8080
+```
+
+You would replace `8080` with whatever port you're running the tests on, and then this should list a PID that you can kill with `kill <pid-here>`.
+
+Please note that this is considered a bug, but it's a known one, and it will be addressed soon!