Well two things come to mind. The first is yeast strains are rated at a certain temperature spread. If you go too low, they go dormant and too high they have trouble as well (and way too high and they all die...but this is much higher than any fermentation attempts I have ever heard of). So that is the first hurdle. Now, you can push them usually with no ill effects outside of the 'acceptable' temperature range but must be willing to accept what that means in terms of the finished product. At lower temperatures you will suppress the formation of fruity esters, resulting in the 'cleaner' lager like taste. Keep in mind yeast activity generates a substantial amount of heat from friction which can will actually raise the temperature of your wort. So your 50 degree ambient air is most likely lower than your actively fermenting wort, albeit this might not be much especially if you are stressing an Ale yeast down to where it is not comfortable. Although I have yet to hear of off flavors as a result of this. (You do hear of it with warmer-than-optimal-ferment temps though). In general you want a clean, fairly fast primary fermentation with Ales but I don't think the too cold will hurt it all that much...but it will take longer.
Fwiw, I pushed a pack of Safale 33 down below the rated range (quite a bit actually). The result was that I thought I had missed it and in a panic (before checking the gravity...doh!) I repitched another pack. I was really freaked when that seemed to just sit there. Well as it turns out, from the moment I started panicking it was probably 65% attenuated. The yeast had done its job just fine. I did however get a very low ester Ale.
