I was digging through some old notes, and came across a simple but useful tip that Rich Garzaniti gave me.

Many applications use resource pools - a good example is connection pools for database connections. Teams often get into trouble because people forget to release the resource back to the pool when they are done with it. As a result you get resource leaks.

To help spot these in testing, make sure you set the resource pool size to 1 in your tests. That way if some code doesn't release its resource the next test that uses that resource will fail, and you don't have too far to look for the culprit.