ISSUE SUMMARY
It is possible to create multiple _id indexes on a collection using ensureIndex({_id:-1}).

db.foo.ensureIndex({_id:-1})
db.foo.ensureIndex({_id:-1})
db.foo.ensureIndex({_id:-1})
db.foo.getIndexes().length 
4

Although having multiple _id indexes on a collection is not intended behavior, the collection may be usable in this state. However, an attempt to subsequently drop such a collection corrupts the namespace:

db.foo.drop()
Fri Aug  8 16:43:48.593 drop failed: {
    "nIndexesWas" : 4,
    "errmsg" : "exception: drop: dropIndexes for collection failed - consider trying repair  cause: BSONObj size: 0 (0x00000000) is invalid. Size must be between 0 and 16793600(16MB) First element: EOO",
    "code" : 12503,
    "ok" : 0
} at src/mongo/shell/collection.js:383

USER IMPACT
The affected namespace is corrupted upon collection drop, and cannot be used until it is repaired.

WORKAROUNDS
Create _id indexes with ensureIndex({_id: 1}) instead of ensureIndex({_id: -1}). If the indexes have already been created and the affected collection needs to be dropped, run db.repairDatabase() afterwards to fix the namespace corruption.

AFFECTED VERSIONS
MongoDB 2.4 production releases are affected by this issue.

FIX VERSION
The fix is included in the 2.4.11 production release.

RESOLUTION DETAILS
Reject ensureIndex({{_id:-1}) requests if the _id index already exists.