It's unpredictable in the sense that even with a compile-time check that the function I'm calling is pure, I don't have any guarantee that it won't throw on my

It's unpredictable in the sense that even with a compile-time check that the function I'm calling is pure, I don't have any guarantee that it won't throw on my particular inputs. This comes up a lot in the Haskell community -- code outside IO is guaranteed by the compiler to be pure, but you can still get hit with an asynchronous exception because you called a partial function.