I have a model with two parts and two mates.  A simple pin with a shoulder in a hole:

The model has two configurations.  The coincident mate has a different mate alignment in each configuration, which I am pretty sure is supposed to be impossible:

When I switch configurations, the model dutifully responds.

The alignment of the concentric mate is the same in both configurations:

The behavior becomes very unstable if you start making changes to the block dimensions or the hole locating dimensions.

These parts were originally in a real model with rebuild errors.  I wonder how many other models have this problem that I don't know about.