A bug I've seen more than once is: a function is passed what it thinks is an immutable list, expects to be able to iterate over it repeatedly and get the same items in the same order, but what it actually has is a read-only view backed by a list that is being mutated by a parallel thread. Another bug I've seen repeatedly is a function that assumes it's passed a sorted list, but actually gets passed an unsorted list.

The problem with structural typing isn't that completely unrelated things end up sharing a type. It's that you have no way to express the subtle distinctions between similar but different things, which are actually when having distinct types is most important.