Using MarkerAxisBand from multiple threads, I get a ConcurrentModificationException. The reason is that accesses to the 'markers' list are not synchronized, so that addMarker() and hashCode() can concurrently read and modify it. I attach a minimal test case to reproduce the problem with JFreeChart 1.0.13.
I'm not 100% sure whether MarkerAxisBand is supposed to be thread-safe. I assume that it should be because the developer guide for version 1.0.4 mentions that JFreeChart should eventually be thread-safe. Also, a similar error got fixed a while ago (bug 757027).
Thanks for fixing this issue!