I have noticed an issue when running in hosted mode and using the Dispatcher. For some reason, hosted mode is able to produce a congruent modification exception as seen in this stack trace:
at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
at java.util.AbstractList$Itr.next(Unknown Source)
This looks to be a problem with hosted mode in the GWT compiler, something with their internal timers being incorrect I would imagine.