I'm fine with making this behavior a different function, if we can come up with a good/clear/concise name for both (I don't think `runError` fits the bill).

However, I don't feel that strongly that this is a problem. I think it'd be pretty weird to assume that your inputs to a function couldn't affect the computational complexity, unless the complexity is documented - witness the `sorted` option that underscore uses in some places, where they use completely different algorithms internally for the same function.