Say I have a marker A which has parents B and C. Say marker B has the parent A. Now suppose I'm checking if A is an instance of C. Depending on which order we added B and C to A, we may recurse down the B path first which would give us A ad infinitum.
Now the way the API Markers are written shouldn't allow such a scenario to happen. However, such a scenario may be possible with user-defined Markers. Due to the theoretical nature of this problem, I think this is a rather minor issue.