Problem with $in and $or filters

[actopolus]

How to reproduce?

1. Add 100K docs to collection
2. Query docs by _id with $in or $or and put 100 idents to expression like this:
   db.collection.find({"_id": {"$in": ["id1", ..., "id100"]})

Expected: 100 docs are returned
Result: unpredictable result, sometime 60 docs, sometime 80, and etc...