I'm not sure if there's a succinct log snippet showing where this is erroring.

It's breaking in windows and Linux, but seems to be limited to debug builds.

Here are some examples of failures in MCI:

http://mci.10gen.com/ui/task/mongodb_mongo_master_windows_64_debug_01afc9b2b1290f16c10c8d591affbdbe157a6d18_13_10_31_13_59_06_aggregation_windows_64

http://mci.10gen.com/ui/task/mongodb_mongo_master_linux_64_debug_duroff_cc5320ccccb6faf2765ded034daee479abafc284_13_10_31_14_36_06_aggregation_linux_64

Buildlogger:
http://buildlogs.mongodb.org/mci_0.9_linux_64_debug_duroff/builds/4777/test/aggregation_0/testshard1.js

I think this was introduced by this commit:
http://github.com/mongodb/mongo/commit/195c99e44a1483816b3be2a1f6f42bffed7ec18e

Here's the oldest recent occurrence in MCI of this particular test failing:
http://mci.10gen.com/ui/task/mongodb_mongo_master_linux_64_debug_duroff_195c99e44a1483816b3be2a1f6f42bffed7ec18e_13_10_31_10_24_07_aggregation_linux_64