I usually apply Isolated attribute on a test class level, so I don't need to remember to mark individual tests as Isolated. But when I started using new TestCase feature of NUnit 2.5 (ability to run multiple parameterized tests using single test method), I had to apply Isolated attribute to individual tests, otherwise some tests failed. It looks like it has to do with a test runner and the order of test execution - often there is no clear pattern of what makes tests fail. But there is a clear pattern of how to fix them: apply Isolated attribute to a method that uses TestCase, even though the whole test class is marked as Isolated.
To me it looks like a bug in TypeMock Isolator related to isolation of test cases - expectations on them are not properly cleared when multiple test cases are invoked from a single method with a help of TestCase attribute.