How is *it++ valid for output iterators?

In example code, I often see code such as *it++ for output iterators. The expression *it++ makes a copy of it, increments it, and then returns the copy which is finally dereferenced. As I understand it, making a copy of an output iterator invalidates the source. But then the increment of it that is performed after creating the copy would be illegal, right? Is my understanding of output iterators flawed?