wrote
That would be true if it were a contraction (do not = don't), but it's
an abbreviation, not a contraction. You can have a "typo" or multiple
"typos".

It is a contraction. "S" being the last letter of "errorS"


Not according to this
http://en.wikipedia.org/wiki/Typographical_error

They do not us an apostrophe at all and use typos instead.

A typographical error (often shortened to typo) is a mistake made in,
originally, the manual type-setting (typography) of printed material, or
more recently, the typing process. The term includes errors due to
mechanical failure or slips of the hand or finger, but excludes errors of
ignorance.[1] Before the arrival of printing, the "copyist's mistake" or
"scribal error" was the equivalent for manuscripts. Most typos involve
simple duplication, omission, transposition, or substitution of a small
number of characters.
Typos are common on the internet in chatrooms, Usenet and the World Wide Web
and some, such as "teh", "pwned", and "pron" have become in-jokes among
Internet groups and subcultures.[4]