Mongo client can't access collections prefixed with an underscore

I don't see the problem other than the shell cannot address this with it's default helper syntax. You can still access collections named like this in the shell:

db.createCollection("_assignments")
{ "ok" : 1 }
db.getCollection("_assignments").find()
db.getCollection("_assignments").insert({ "a": 1 })
WriteResult({ "nInserted" : 1 })
db.getCollection("_assignments").find({ "a": 1 })
{ "_id" : ObjectId("53a36a4047234c4e9bb4feac"), "a" : 1 }

This is a known bug. Don't prefix collections with underscores.

https://jira.mongodb.org/browse/SERVER-445