Oddly enough, errors do occur - Trinity College guitar grade exams come to mind. I spotted it (as teacher) and advised the examiner that my pupils would be playing a corrected edition, which the examoner agreed to. Point is, unless you're playing outlandish modern stuff, most errors are pretty obvious as such.
On the other hand, the collectors, Sharp, Hammond and Gardner et al., were notorious for taking stuff down errors and all, resulting in some of the stranger tonalities and nonsensical words we hear in traditional song clubs from time to time...How do you deal with that one?
KYBTTS