So-called input iterators (in terms of C++) are iterators that can only be advanced forward and cannot be saved to iterate over the same elements one more time. Some methods (is_null, cycle, drop_while) save old state to iterate from it later, so they don't work properly with them. For example, is_null copies the state and then advances the iterator with copied state to see if iterator yields something. The problem is, once you advanced an "input" iterator, it can never be advanced back, so the fact that you copied state doesn't really help.

Examples of "input" iterators: iterator reading from console/socket, database cursor.