When creating an index, createIndex will validate that the index option keys are valid. The following index definition is not valid because there is no spork option:

> db.COLL.createIndex({a:1},{spork:true})
{
  "ok": 0,
  "errmsg": "The field 'spork' is not valid for an index specification. Specification: { key: { a: 1.0 }, name: \"a_1\", spork: true }",
  "code": 197,
  "codeName": "InvalidIndexSpecificationOption"
}

However an invalid option value for a known option key will succeed:

> db.COLL.createIndex({a:1},{sparse:"PanGalacticGargleBlaster"})
{
  "createdCollectionAutomatically": true,
  "numIndexesBefore": 1,
  "numIndexesAfter": 2,
  "ok": 1
}

This can cause problems for other tooling that assumes sparse will always be a boolean value. Even more insidious is in cases such as sparse:1 rather than sparse:true.

The same problem can be observed with background and other index options.