Robert Lucas, a godfather of the rational expectations movement has come in for a lot of criticism recently. Here's my own brief attempt.
Imagine an artificial economy with just two actors, who each have to guess game theory style at the likely behaviours, economically, of the other. They both are fully cognisant of each other's economic models. All they need to do is apply those rules to apply a decent best guess of model parameters - the legendary sloppy assumptions - and we'll sit back and watch well known macro-economic phenomena emerge from their identically specified micro-level models of themselves as economic actors.
Now lets imagine those models shared a similar property (as many many models do) with the logistic function, $X(n+1)=rX(n)(1-X(n))$, namely they are riddled with chaos. Our two agents might agree perfectly on each other's model and what's more be correct but when it comes to apprximating the model parameters, needless to say, they cannot guess the other's starting value with infinite precision. The result, over certain wide ranges of the parameter phase space - is utter chaos. All it takes for rational expectations to be shown to be inadequate is some likelihood of such radical non-linearity in real (no pun intended) sets of micro-founded models of agent interactions within the wider rational expectations movement.
