In Haskell, type error messages are always like "expected types A and B to be equal, but they are not". The problem is that, without type annotations, the compiler cannot know if it is A or B that is wrong, which can result in confusing error messages.

For example, suppose that you have a bug in the body of a function, but did not provide a type annotation for it. The function might still compile but not with the type you want. The compiler will only notice something is amiss when you try to call the function and it turns out that the function's inferred type doesn't fit the call site.

Basically, global type inference in the absence of type annotations means that changes in one part of the file can affect inferred types very far away. In practice it's best to use type annotations to limit inference to small sections, so that type errors are reported close to what caused them.