On a side note this is known as a "Persian Flaw". Persian carpet makers would always leave a deliberate flaw as an acknowledgement that man is fallible and only God is perfect. The habit also served to uniquely identify the maker and to make it harder to copy. British banknotes carry a persian flaw as a counter measure against counterfeiting. Some authors also use the technique with a deliberate spelling mistake. The British Intelligence Services have a different persian flaw on every memo distributed so should information be leaked they know who it came from. This technique can be handy when confiding in friends. Make a small change for each friend and then if somebody breaches the confidentiality you know who it was.