The only problem with IEnumerable's open-endedness is that it's so open-ended. It makes no implicit guarantees about order, finiteness, side effects, speed/effi

The only problem with IEnumerable's open-endedness is that it's so open-ended. It makes no implicit guarantees about order, finiteness, side effects, speed/efficiency, idempotency etc. It's easy to assume those things until you accidentally find a situation where one or more are not in your favor.