Substituting margarine for butter in cookies
I have a family member who is lactose intolerant, and we often substitute margarine (or various oils) for butter in cooking. We've found that this is mostly interchangeable, but can be unpredictable in baking due to different melting properties. Also, as Marti noted in a comment below, it's a challenge to find unsalted margarine (important if directly substituting for unsalted butter).

I'd love to see any answers on strategies for adapting recipes to account for butter-margarine differences, but in my experience it is easier to either settle for slightly sub-par cookies — or (preferably) to try non-dairy or vegan recipes instead, which are designed with a non-butter fat in mind and therefore have better results.