There are a number of different kinds of aliasing that can occur in BIST engines. The most commonly discussed type of aliasing occurs when two faults have a cancelling effect in the signature register. In other words, two wrongs appear to make a right. Two other forms of aliasing are zeroing and overcounting the LFSR. Zeroing refers to the occurrence of a zero in the LFSR that can have a reset-like effect on the LFSR and 'erase' the history of the previous tests. Overcounting occurs when a value or sequence in the LFSR occurs more than once in a test, typically due to a roll-over (e.g. 1 + 2n cycles of an n-bit counter will guarantee that at least one value will be repeated). Overcounting can generate a similar test history 'erasing' phenomenon.