Testing your test cases

What is a good practice for testing test cases, checking them for false positives?

For example, I am writing a mission-critical class in JavaScript. To make sure every little nook and cranny is covered, we have well over 300 unit tests for it. From time to time, the code changes, breaks--and one or two tests continue to come back positive--usually because of JavaScript's willingness to convert various data types to boolean.

Are there well-worn patterns out there in the world used by great QA teams to hunt for false positives?