If you import a contact who has a 10 digit number (becuase of the extension) and then attempt to seperate the extention you will find this bug:

It comes in looking like:

1234567890211

When you put a space between the 0 and the 211 it places an x there because it knows its an extension but it also cuts off the last number and replaces it with an x as well. The end result is:

(123)456-7890 x21x

Thanks for looking into this!