We ran into a situation where the memstore size became negative due to expensive postPut/Delete Coprocessor calls in doMiniBatchMutate. We update the memstore size in the finally block of doMiniBatchMutate, however a queued flush can be triggered during the coprocessor calls(if they are taking time eg. index updates) since we have released the locks and advanced mvcc at this point. The flush will turn the memstore size negative since the value subtracted is the actual value flushed from stores. The negative value impacts the future flushes amongst others that depend on memstore size.