IPhone : XCTest pass in isolation, fail when run with other tests

When I run a single XCTest class, all tests within succeed.

However when I run it together with other XCTest, some tests in the class fail.

setUp and tearDown method are implemented correctly as following:

- (void)setUp {
  [super setUp];
  ...
}

- (void)tearDown {
  ...
  [super tearDown];
}

Does anyone know why this might happen?

Thanks a bunch!