I was working with a stable installation of an elaborated platform. Some plugins were installed. After upgrading the installation by 2 patch level the customer registration was not working anymore.

In these two patch level a method in an interface got an additional optional parameter which had a major impact on the behaviour the implemented method. A plugin decorated the implementation without knowing about the new parameter. Therefore when calling the method the decorating class did not pass the new parameter in to the decorated implementation and the fallback value was given instead.

The caller expected the method to do something and did not branch into an alternative way but the default value disables the expected behaviour. Eventually nothing happened.

Breaking changes in patch levels woop di fucking do.