You've got to remember that even when homebrewers make starters and all that, we're almost always STILL underpitching, by commercial brewers' standards anyway. Most of us could probably safely double (if not triple, quadruple, or more) the size of our typical starter and still be perfectly fine, but making gigantic starters is more expensive, and not necessary enough to bother with unless you're dealing with a huge beer where you need all the yeast you can get.
From the White Labs FAQ:
An often quoted number is to pitch 1 million cells/ml/degree Plato of beer, which equals about 250 billion cells for 5 gallons. That is okay, more cells are not detrimental until about 400 billion cells.
400 billion cells is a lot. You'd probably need to pitch at LEAST four liquid yeast packs/vials, assuming they were totally fresh, or way more of them if they were a month or more old. Even with dry yeast, you'd likely need 3-4 packs, and that's assuming they're in good shape and you rehydrate for maximum viability.
As david_42 says, pitching on a yeast cake seems to be the only likely way for a homebrewer to overpitch. If you put a big beer on a smaller beer's yeast cake I think you're almost certainly fine. If you were trying to use a yeast cake from a bigger beer on an identical (or smaller) beer, you may want to start thinking about removing some of the yeast cake, but I don't think that's a huge concern, especially if you wash your yeast cake, which involves the loss of some of the yeast anyway.
