Nothing like this has ever happened to Nadal since he won Roland Garros for the first time.
Before the claycourt season I looked up his career results and noted all the active player who had ever beaten him on clay, a list that began and ended with Federer, Djokovic, Ferrer (very early in Rafa's career), Verdasco (blue clay), Zeballos (first tourney back from long injury) and Volandri (in a challenger). Suddenly, he's lost back-to-back to Ferrer and Almagro with no excuses.
If Djokovic were healthy, I'd be confident Nadal's run at Roland Garros was about to end. But other than Djokovic, it's still hard for me to imagine any player capable of beating Rafa on clay in a best-of-five set match. Certainly not Ferrer or Almagro, both of whom almost blew their three-set wins against him by getting tight.