I recently made a class implementing an interface from a library in python. That interface had a "canonical" implementation, in that most instances of that interface were of that class. When I tried to call some functions from that library with my own class, they tried to call methods from the "canonical class", which were neither present in the interface nor my implementation.

Result: program crashed when trying to call not present method. Had to clear up a few of these before it would work. If the type system hadn't been so forgiving the library implementers would have realized how their abstractions were leaking, and it would have been a lot easier for me to implement the interface correctly.