Take the following update:

db.test.update(
  {"someField" : {"$all": ["a", "b"]}}, 
  {"$addToSet": {"someOtherField": 123}, 
    "$setOnInsert": {"someField": ["a", "b"]}},
  {"upsert": true})

This results in the following error, because of the $all query:

"writeError" : {
  "code" : 54,
  "errmsg" : "cannot infer query fields to set, path 'someField' is matched twice"}

Given that I explicitly stated what I want the value for someField to be using $setOnInsert, it doesn't have to be a problem that the value can't be inferred from the query. It could just use value I specified as a fallback for inference, or even instead of inference.