This sort of mistake gets harder and harder to prevent as programs get larger. For example, one programmer might use a non-atomic Rc pointer to hold a string in some random Error value. Later, another programmer might try to parallelize that part of the program, without realizing that sending an Error across a thread boundary is now illegal. Or maybe the same mistake happens in the other order.

Here's Niko Mastakis talking about a case where that happened to him (I think): https://youtu.be/lO1z-7cuRYI?t=4m