Ran accidentally map reduce to a busy server and had to kill the client process as some client connections were not responding anymore. After killing all processes the temporary collection '_purge' was not removed and even after cleanly shutting down and restarting mongo it can not be removed.

ubuntu@db:~$ /opt/mongodb/bin/mongo
MongoDB shell version: 2.4.4
connecting to: test
> use mydb
switched to db mydb
> show collections
_purge
adminmessages
...

> db._purge.drop()
Sat Jan 18 06:12:06.024 JavaScript execution failed: TypeError: Cannot call method 'drop' of undefined
> db['_purge'].count()
Sat Jan 18 06:12:26.334 JavaScript execution failed: TypeError: Cannot call method 'count' of undefined
> db.getCollectionNames();
[
"_purge",
"adminmessages",
....
Can see this from mongo log

Sat Jan 18 04:50:54.451 [conn5226] CMD: drop war._purge
Sat Jan 18 04:50:54.566 [conn5226] command war.$cmd command:

{ drop: "_purge" }

ntoreturn:1 keyUpdates:0 reslen:117 140ms
Sat Jan 18 04:50:54.695 [conn5226] command admin.$cmd command:

{ renameCollection: "war.tmp.mr.battleinbox_118", to: "war._purge", stayTemp: false }

ntoreturn:1 keyUpdates:0 reslen:37 120ms
Sat Jan 18 04:50:54.714 [conn5226] CMD: drop war.tmp.mr.battleinbox_118
Sat Jan 18 04:50:54.761 [conn5226] CMD: drop war.tmp.mr.battleinbox_118
Sat Jan 18 04:50:54.761 [conn5226] CMD: drop war.tmp.mr.battleinbox_118_inc
Sat Jan 18 04:50:54.849 [conn5173] update war.inbox query:

{ read: false, uid: 202584 }

update: { $set:

{ read: true }

} nscanned:3 nupdated:2 keyUpdates:0 numYields: 1 locks(micros) w:783 107ms
Sat Jan 18 04:50:54.849 [conn4886] insert war.mapevents ninserted:1 keyUpdates:0 locks(micros) w:435 108ms
Sat Jan 18 04:50:54.903 [conn5145] update war.battleinbox query:

{ read: false, uid: 212933 }

update: { $set:

{ read: true }

} nscanned:39 nupdated:21 keyUpdates:0 numYields: 2 locks(micros) w:101363 186ms
Sat Jan 18 04:50:54.920 [conn5226] CMD: drop war.tmp.mr.battleinbox_118
Sat Jan 18 04:50:54.931 [conn5226] CMD: drop war.tmp.mr.battleinbox_118_inc
Sat Jan 18 04:50:55.131 [conn5136] query war.users query:

{ gcid: "G:625654150", account_type: 6 }

ntoreturn:1 ntoskip:0 nscanned:112460 keyUpdates:0 numYields: 251 locks(micros) r:1113617 nreturned:0 reslen:20 1933ms
Sat Jan 18 04:50:55.135 [conn5226] command war.$cmd command: { mapreduce: "battleinbox", map:
function()

{ emit(this.uid, 1); }

, reduce:
function(key, values)

{ return Array.sum(values); }

, out: "_purge" } ntoreturn:1 keyUpdates:0 numYields: 69182 locks(micros) W:302844 r:1682304504 w:28452182 reslen:130 1952453ms

...
Sat Jan 18 04:53:18.628 [conn5226] getmore war._purge cursorid:3953529293811394301 ntoreturn:0 k
eyUpdates:0 numYields: 23 locks(micros) r:119699 nreturned:56952 reslen:1879436 134ms