A closer look at intelligibility failures shows that some phoneme confusions are more probable than others, since stimulus and response have a larger number of phonetic features in common. Since they can partly also be found in natural speech, these failures indicate consequently not solely a quality loss of synthetic or degraded natural speech but, moreover, they can also be a mirror image of human speech in general. Consequently, for the assessment of, e.g., speech output systems, those confusions that can be observed frequently in natural speech as well have to be weighted less than those ones that occur very seldom.