I was going to put a whole back-story for this question here, but thought it'd be better as a Google+ post instead, for reasons that should be obvious to anyone who reads it.
So, during high velocity returns, is there a chance that a parachute will fail if the capsule is travelling too fast when it's deployed? If so, what's the optimal speed/altitude range for deployment?
If speed is not the cause of these failures, what is, and how can I mitigate it?
I should also say that I'm using Mechanical Jeb. For reasons explained in this question, I now always bring the MechJeb unit back with me. In the case of the most recent failure, the MechJeb unit was positioned between the capsule and the parachute, something that had worked for me before. But for my rendezvous mission, I used one of the radial mount options, keeping the (proper sized) parachute connected to the top of the capsule.