While testing sharding I found a scenario that reliably crashes the mongod server. Really enjoying my evaluation with mongodb. Best of luck!
~K

Here's my setup:
mongod --shardsvr --dbpath /Users/kae/dev/shard-test/1/db --port 40000
mongod --shardsvr --dbpath /Users/kae/dev/shard-test/2/db --port 40001
mongos --configdb localhost:40000

To recreate the crash,
browse to: http://localhost:41000/config/chunks/

EXPECTED:
I was hoping view information about chunk usage.

ACTUAL:
The mongod referenced crashes with the message:

terminate called after throwing an instance of 'mongo::MsgAssertionException'
what(): Cannot create a properly formatted JSON string with element: lastmod: Timestamp 1263785266000|1 of type: 17
Sun Jan 17 19:39:47 Got signal: 6 (Abort trap).
Abort trap