I vaguely recall from a podcast that had some of the White Labs people on it, they said that overpitching isn't a big deal as long as you don't harvest the yeast afterward. I don't recall the reason, I think it had to do with the amount of dead yeast that were starved from not enough resources.
I do not thing that this is correct.
OK, I found where I heard that. Wrong about the reason, but right about it being ok as long as you don't harvest it.
If you go to the 2012 BJCP seminars (
http://www.homebrewersassociation.org/pages/lets-brew/homebrewing-seminars/2012) and listen to Neva Parker's talk about "Fermentation Mythbusters", skip to about 28:20 in and she mentions why you shouldn't use too much yeast. Not because they are dead or starved for resources, something about yeast growth and new yeast generation for subsequent generations. So, as long as it's the last time you use the yeast you can over pitch, but don't harvest that yeast when done.