Complex sentence: A third possibility is that a signed integer is being used, the overflow could not only result in an incorrect value being returned, but that the value may be reported incorrectly as an unexpectedly negative or positive value.
Split into simpler sentences: A third possibility is that a signed integer is being used. In such a case, the overflow could not only result in an incorrect value being returned, but that the value may be reported incorrectly as an unexpectedly negative or positive value.