I made the experience that I sometimes rely on my tests too much when it comes to the design of the system. I am basically too low in the nitty-gritty implementation details to take the step back to look at the bigger picture. This often results in an unnecessary complex design. I know, I am supposed to refactor the code but sometimes I have the impression that I could save a lot of time by taking the step back more often.
That being said, if you have a framework like rails where your architectural decisions are very limited these problems are basically non existent.
Another problem is when you trust your tests blindly. Truth is - like any other code - your tests can have bugs too. So be as critical towards your tests as you are towards your implementation.