Many people in baseball believed that Pettitte had some kind of elbow problems years before the Yankees let him go.
The Astros just didn't do their due diligence when it came to signing Pettitte. Luckily, it also netted them Clemens, otherwise that signing would have been a total disaster.
"And so he spoke, and so he spoke, that lord of Castamere. But now the rains weep o'er his hall, with no one there to hear." - The Rains of Castamere