If for some reason you create a {_id:1} index on capped collection, there is no way to remove it.

both dropIndex("id") and .dropIndexes() fail

> db.testCapped.dropIndex("id")

{ "nIndexesWas" : 1, "errmsg" : "may not delete _id index", "ok" : 0 }

> db.testCapped.dropIndexes()
{
"nIndexesWas" : 1,
"msg" : "non-_id indexes dropped for collection",
"ok" : 1
}