Certificate of Correction
a.    Easiest case: Obvious typo. Specification says “higf” instead of “high”. Point it out, gets fixed. Free if PTO’s mistake, honest mistake modest fee.
b.    Harder case: Typo that isn’t clear. Know its a typo but don’t know what correct answer should be. pH from Z to 9. If you are trying to broaden your claim w/o adequate notice, can’t use this mechanism. This is really aimed at clear typos where you know what it should read.
c.    Hardest case: Typos that aren’t obvious at all, pH of 3-9, you meant 6-9. People will rely on typo, think patent is invalid and infringe.