This may affect more versions, but I've only tested on 2.2.0 and 2.0.0.

Trying to do an upsert with a $not modifier in the find:

db.counters.update({"_id" : {a: 5, b:10}, "$not":{"value.tx":"5d"}}, {"$inc":{"value.c":4}, "$set":{"value.tx":"5d"}}, true)

Works for the first insert. But the second always fails with the following exception, no matter what $not is set to:

E11000 duplicate key error index: databaseone.counters.$_id_  dup key: { : { a: 5.0, b: 10.0 } }