The MPEP specifies when a rejection can be made final:

A second Office Action on the merits should be final, except where the examiner introduces a new ground of rejection that was not necessitated by either amendment or an information disclosure statement.  This is in accord with the philosophy espoused by the Federal Circuit in In re De Blauwe, 736 F.2d 699, 706 n.9, 222 USPQ 191, 197 n.9 (Fed. Cir. 1984), where the court stated that when the Office advances “a position or rationale new to the proceedings, an applicant must be afforded an opportunity to respond to that position or rationale by submission of contradicting evidence.”