One thing that Kent Beck mentioned in Test Driven Development was that if he's not finished test-driving a chunk of code, but has to leave it for a while, he'll leave one test failing. When he gets back to pick up where he left off, he runs the tests, sees the failing one, and by making that test pass, gets back into the mindset needed for that design problem.
I do this all the time. This is especially handy when you are programming at home and are susceptible to frequent family interruptions. It's also handy when you are done for the day. When you come back in the morning, the test is there ready to tell you exactly where you left off.