They seem to not have been added to the nsdetails hash.

> db.getCollectionNames()
"tmp.mr.type_foo_tmp_reduce_items_45993_47129_inc",
"tmp.mr.type_foo_tmp_reduce_items_45994_47130_inc",
"type_foo"
]

> db.getCollection("tmp.mr.type_foo_tmp_reduce_items_45994_47130_inc").stats()
{
"ns" : "indexer_cache.tmp.mr.type_foo_tmp_reduce_items_45994_47130_inc",
"sharded" : false,
"primary" : "p",
"errmsg" : "ns not found",
"ok" : 0
}

> db.getCollection("tmp.mr.type_foo_tmp_reduce_items_45994_47130_inc").drop()
false

– ON RS

PRIMARY> db.oplog.rs.findOne(

{"o.drop": "tmp.mr.type_foo_tmp_reduce_items_45994_47130_inc"}

)
null

From the log:

Mon Aug 8 08:23:59 [conn50679] CMD: drop dbname.tmp.mr.type_foo_tmp_reduce_items_45925_47047
Mon Aug 8 08:23:59 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:23:59 [conn50679] CMD: drop dbname.tmp.mr.type_foo_tmp_reduce_items_45925_47047_inc
Mon Aug 8 08:23:59 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:23:59 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:23:59 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:23:59 [conn50679] mr failed, removing collection
Mon Aug 8 08:23:59 [conn50679] CMD: drop dbname.tmp.mr.type_foo_tmp_reduce_items_45925_47047
Mon Aug 8 08:23:59 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:23:59 [conn50679] CMD: drop dbname.tmp.mr.type_foo_tmp_reduce_items_45925_47047_inc
Mon Aug 8 08:30:00 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:30:00 [conn50679] CMD: drop dbname.tmp.mr.type_foo_tmp_reduce_items_45934_47056
Mon Aug 8 08:30:00 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:30:00 [conn50679] CMD: drop dbname.tmp.mr.type_foo_tmp_reduce_items_45934_47056_inc
Mon Aug 8 08:30:00 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:30:00 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:30:00 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:30:00 [conn50679] mr failed, removing collection
Mon Aug 8 08:30:00 [conn50679] error: hashtable namespace index max chain reached:1335
Mon Aug 8 08:30:00 [conn50679] CMD: drop dbname.tmp.mr.type_foo_tmp_reduce_items_45934_47056
Mon Aug 8 08:30:00 [conn50679] error: hashtable namespace index max chain reached:1335