> x
{ "_id" : 0, "a" : [ [ { "a" : [ 1 ] } ] ] }
> db.mytest.insert(x)
SingleWriteResult({
        "writeErrors" : [ ],
        "writeConcernErrors" : [ ],
        "nInserted" : 1,
        "nUpserted" : 0,
        "nUpdated" : 0,
        "nModified" : 0,
        "nRemoved" : 0,
        "upserted" : [ ]
})
> db.mytest.find({"a.0": {$elemMatch: {a: 1}}})
{ "_id" : 0, "a" : [ [ { "a" : [ 1 ] } ] ] }
> db.runCommand({buildinfo: 1})
{
        "version" : "2.5.5-pre-",
        "gitVersion" : "4e62d347f090ce1afd91dc59b3db2fab8f930bc0",
        "OpenSSLVersion" : "",
        "sysInfo" : "Darwin Andrew-Emil-MacBook-Pro.local 12.4.0 Darwin Kernel Version 12.4.0: Wed May  1 17:57:12 PDT 2013; root:xnu-2050.24.15~1/RELEASE_X86_64 x86_64 BOOST_LIB_VERSION=1_49",
        "loaderFlags" : "-fPIC -pthread -Wl,-bind_at_load -mmacosx-version-min=10.6",
        "compilerFlags" : "-Wnon-virtual-dtor -Woverloaded-virtual -fPIC -fno-strict-aliasing -ggdb -pthread -Wall -Wsign-compare -Wno-unknown-pragmas -Winvalid-pch -Werror -pipe -O3 -Wno-unused-function -Wno-deprecated-declarations -mmacosx-version-min=10.6",
        "allocator" : "tcmalloc",
        "versionArray" : [
                2,
                5,
                5,
                -100
        ],
        "javascriptEngine" : "V8",
        "bits" : 64,
        "debug" : false,
        "maxBsonObjectSize" : 16777216,
        "ok" : 1
}

> x
{ "_id" : 0, "a" : [ [ { "a" : [ 1 ] } ] ] }
> mydb.mytest.insert(x)
> mydb.mytest.find({"a.0": {$elemMatch: {a: 1}}})