Reported by Joe.
It is is known that resolving a shift/reduce conflict in favor of reduction removes a transition in the automaton and therefore can make some states unreachable. A conflict in an unreachable state is declared unexplainable. This should be fixed: either unreachable states should be removed entirely, or the conflicts in these states should not be reported at all.
I have not yet confirmed whether these unexplainable conflicts are indeed found in unreachable states.