On a common formulation, rationalist infallibilism is committed to two main theses: (i) ‘analytic a priori infallibilism’ and (ii) ‘synthetic a priori infallibilism’. According to thesis (i), a relatively wide range of analytic a priori propositions can be infallibly justified. According to thesis (ii), a relatively wide range of synthetic a priori propositions can be infallibly justified. In this paper, I focus on rationalist infallibilism’s second main thesis, what is being called ‘synthetic a priori infallibilism’. I argue that synthetic a priori infallibilism, and by extension rationalist infallibilism, is untenable. In particular, exploring what seems to be the only potentially plausible species of synthetic a priori infallibility, I reject the infallible justification of propositions about the self.