CappedIterator verify failing in parallel tests

Several MCI builders have started failing the 'parallel' suite. First failing builds at hash https://github.com/mongodb/mongo/commit/9c57a94477344706c357fb11f05c12e2403a1b91, though there are several inactive commits preceding.

Failing MCI builds:

http://mci.10gen.com/ui/task/mongodb_mongo_master_linux_64_9c57a94477344706c357fb11f05c12e2403a1b91_13_08_16_13_30_06_parallel_linux_64

http://mci.10gen.com/ui/task/mongodb_mongo_master_linux_64_debug_9c57a94477344706c357fb11f05c12e2403a1b91_13_08_16_13_30_06_parallel_linux_64

http://mci.10gen.com/ui/task/mongodb_mongo_master_linux_64_duroff_9c57a94477344706c357fb11f05c12e2403a1b91_13_08_16_13_30_06_parallel_linux_64

http://mci.10gen.com/ui/task/mongodb_mongo_master_linux_64_debug_duroff_9c57a94477344706c357fb11f05c12e2403a1b91_13_08_16_13_30_07_parallel_linux_64

http://mci.10gen.com/ui/task/mongodb_mongo_master_rhel_57_64_bit_9c57a94477344706c357fb11f05c12e2403a1b91_13_08_16_13_30_07_parallel_rhel57