> db.demo.insert({foo:{bar:1}})
> db.demo.find({},

{foo:1}

)
{"_id" : ObjectId( "4a80641c253e1a428459ca94") , "foo" : {"bar" : 1}}
> db.demo.find({},

{'foo.bar':1}

)
{"_id" : ObjectId( "4a80641c253e1a428459ca94") , "foo" : {"bar" : 1}}
> db.demo.find({},

{'foo.qux':1}

)
error:

{"$err" : "assertion during query"}

> db.demo.find({},

{'baz.qux':1}

)
> db.demo.find({},

{'baz':1}

)
>

Note that the last two don't assert.

Server log:
Mon Aug 10 14:18:03 test.demo Assertion failure !e.eoo() db/jsobj.h 909
0x44ae87 0x45179d 0x444a4d 0x444f3d 0x48f6b9 0x4901eb 0x51e4f6 0x52542b 0x527ed1 0x47ed4c 0x4f8ed0 0x4fbe00 0x53403a 0x545a57 0x54565f 0x3742206367 0x37416d30ad
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x44ae87]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x45179d]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x444a4d]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x444f3d]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x48f6b9]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x4901eb]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x51e4f6]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x52542b]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x527ed1]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x47ed4c]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x4f8ed0]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x4fbe00]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod(_ZN5mongo10connThreadEv+0x1fa) [0x53403a]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x545a57]
/home/fds/pp_web/palogging/mongo/prefix/bin/mongod [0x54565f]
/lib64/libpthread.so.0 [0x3742206367]
/lib64/libc.so.6(clone+0x6d) [0x37416d30ad]
Mon Aug 10 14:18:03 Assertion:
Mon Aug 10 14:18:03 test.demo Caught Assertion in runQuery ns:test.demo massert:
Mon Aug 10 14:18:03 ntoskip:0 ntoreturn:0
Mon Aug 10 14:18:03 query:{}