Hmm @jimmuller, I think we're getting down to semantics. My understanding is that sizing a chain big/big + 2 should be enough chain for most systems. Too short a chain can pose a serious risk.
But say your chain is long enough but you're exceeding the rd capacity by "too much". (5 teeth, 10t, 20t, whatever.) As I see it the main risk here is that the chain will sag and/or rub on the derailleur in small/small -- not a good thing but you and the bike will be fine. But what about "overstretching" the rd in big/big? My thought is that as long as the chain is long enough this shouldn't be catastrophic, though I suppose an rd could break if overstretched too much -- say parallel to the chainstay.
Anyway, for most real world situations (i.e. exceeding rd capacity by -5t) I see no catastrophic risks **as long as the chain isn't too short**.
Last edited by jethin; 01-24-16 at 11:24 AM.