Note the "Safety Inferred" seal of approval on the upper right hand corner of the hackage package http://hackage.haskell.org/packages/archive/lens-family-core... This means it survives a recursive check of dependencies that precludes that kind of nonsense.

I hadn't thought of that. Types serve as a good safety net, but they're not foolproof. Expecting something like that to be correct because it passed through the type checker is like expecting to be served at a restaurant dressed in a gorilla suit because you got through the door.

And of course it is fun to think about total functions that are "effectively" bottom...

    id :: a -> a
    id a = case (a,ack 4 3) of
        (x,0) -> x
        (x,_) -> x
        where
            ack 0 n = n+1
            ack m 0 = ack (m-1) 1 
            ack m n = ack (m-1) (ack m (n-1))