Buying stock in great companies when they are dealing with some short-term issue that creates stock price weakness can set investors up for big gains down the road. As long as the business is still in good shape, adding it to your portfolio when it's trading at a sizable discount can look like a brilliant move in the future.Stocks normally trade at discounts when there are concerns or question marks about the company's future performance. But in the case of Pfizer (PFE 0.77%) and Comcast (CMCSA 0.60%), the ...