Of the few options available during invalidation, the amendment of obvious errors is a notable one. But what makes an error ‘obvious’? Typical examples include obvious ungrammatical phrasing, obvious incorrect use of technical terms, or obvious typographical mistakes. Such an amendment ran counter to the original purpose of the ‘obvious error’ mechanism and risked opening the door to its misuse. That makes the patentee’s correction a textbook case of an ‘obvious error’ that should have been accepted.