Given a newly set up server, configured to run in a replica set, the following steps in the mongo console will hang the server if running on Ubuntu:

rs.initiate()
<wait for the replica set>
use bug
db.map.ensureIndex(

)
db.map.update({loc:{$near:[1.0,2.0],$maxDistance:3.0}, people:{$exists:true}}, {$set:

, $push:{people:"chris"}}, true)

At this point, the mongo database will become non-responsive and consume all available CPU. This appears to be an issue with the $set for the location being performed as an upsert from a distance query. Running on Windows or without a replica set does not appear to cause the problem.