When someone buys a product with Kosher certification, do they have to check to make sure that it is still certified Kosher or can they rely on the fact that the last time (or first time) they bought it it had certification? For how long can someone rely on the previous time they checked?
My family used to use a certain brand of margarine that was Pareve for years. We didn't check the Hechsher (Kosher certification) on the product very often, but one day it changed from being Pareve to being dairy. Luckily, we noticed that it had become dairy and we stopped buying that margarine. Had we not checked, it would very likely have led to mixing of meat and milk.
I recommend always looking for a Hechsher.