A mostly unexpected possibility suddenly became very real again.Just when you think shares of discount retailers Dollar General (DG -5.10%) and Dollar Tree (DLTR -6.53%) can't move any lower, they find a way. The former fell little more than 5% today, while the latter was down to the tune of 6.5%. Both sell-offs -- rooted in the same reason -- carried their respective stocks to new 52-week lows.Might this prolonged and persistent weakness be a buying opportunity for patient investors that can stomach some r ...