It's confusing, isn't it? Errors are displayed red, which looks scary, while failures are displayed yellow, which looks inviting. But, debugging a test failure, you don't get a trace of a clue what caused the failure. While debugging an error, you have the stack trace right there and know exactly where to look.
When SUnit lists yellow and red failed unit tests, I invariably start with the red ones. They're usually easier to fix.