If an unknown pixel spectrum happens to match two reference spectra with the exact same spectral angle value, then the pixel will be assigned to the last tested spectrum. So if you have 8 classes (reference spectra or ROIs for training data), and the spectral angle is identical with class 2 and 5, then it will be assigned to class 5 (assuming class 5 is the 5th listed in the reference/training data set). This might mean your reference spectra are not unique and perhaps represent the same class. This is where the Rule Images can be very useful!