When placing a throughput load of approximately 15 queries per second we can intermittently see a 'java.lang.IndexOutOfBoundsException'.
Upon further investigation it was found that the BoboSearcher2 class is using an instance variable '_facetCollectors' which is not thread safe. This variable is set by a call from the BoboSubBrowser.browse method which could be executed via multiple threads simultaneously.
Changing this instance variable to utilise a ThreadLocal instead addresses this problem.
Please find a patch file attached.
OS X and Ubuntu