In the context of stemmatology, common means ‘shared’ (not ‘simple’ or ‘vulgar’), so 'common errors' is the traditional expression used for 'shared innovations' (cf. the notion error). Usually one means by this expression that these common errors did not arise polygenetically. Stemmata are drawn and rooted by considering common and significant errors only. Witnesses that share only readings that are not common errors (shared innovations) are not closely related as the archetype will already have contained these readings. For further details compare Lachmann's method.
In other languages
DE: gemeinsame Fehler, Methode der Fehlergemeinschaften
FR: (méthode des) erreurs communes
IT: metodo degli errori (comuni)