Your example of the unique sparse index is wrong. Why shouldn't one be able to insert the last two records? Unique index is on score and not on userid!
Here my console output to prove it:
mongos> db.scores.find()
mongos> db.scores.ensureIndex(

{ score: 1 }

,

{ sparse: true, unique: true }

)
mongos> db.scores.insert(

{ "userid": "PWWfO8lFs1", "score": "43" }

)
mongos> db.scores.insert(

{ "userid": "XlSOX66gEy", "score": "34" }

)
mongos> db.scores.insert(

{ "userid": "nuZHu2tcRm" }

)
mongos> db.scores.insert(

{ "userid": "HIGvEZfdc5" }

)
mongos> db.scores.find()

{ "_id" : ObjectId("526a8be3f46d2610cd367984"), "userid" : "PWWfO8lFs1", "score" : "43" } { "_id" : ObjectId("526a8be3f46d2610cd367985"), "userid" : "XlSOX66gEy", "score" : "34" } { "_id" : ObjectId("526a8be3f46d2610cd367986"), "userid" : "nuZHu2tcRm" } { "_id" : ObjectId("526a8be5f46d2610cd367987"), "userid" : "HIGvEZfdc5" }

mongos> db.scores.insert(

{ "userid": "PWWfO8lFs1", "score": "82" }

)
mongos> db.scores.insert(

{ "userid": "XlSOX66gEy", "score": "90" }

)
mongos> db.scores.find()

{ "_id" : ObjectId("526a8be3f46d2610cd367984"), "userid" : "PWWfO8lFs1", "score" : "43" } { "_id" : ObjectId("526a8be3f46d2610cd367985"), "userid" : "XlSOX66gEy", "score" : "34" } { "_id" : ObjectId("526a8be3f46d2610cd367986"), "userid" : "nuZHu2tcRm" } { "_id" : ObjectId("526a8be5f46d2610cd367987"), "userid" : "HIGvEZfdc5" } { "_id" : ObjectId("526a8bf5f46d2610cd367988"), "userid" : "PWWfO8lFs1", "score" : "82" } { "_id" : ObjectId("526a8bf7f46d2610cd367989"), "userid" : "XlSOX66gEy", "score" : "90" }

mongos>