He basically defined existence as thinking, then turned it into a tautology. And then it blew up in his face when he realized that he couldn't prove anything else was thinking. And in fact even if there wasn't some devil tricking him (ala brain in a box hooked to the Matrix), things like rocks and water could never exist.

His entire philosophy was flawed right from the start. But it usually is. That's why so many philosophy texts take the first chapter (or volume) refuting philosophers who came before them.