If one tries to $setOnInsert an array or increment a positional (or several) elements of that array if that document already exists, the instruction fails. Depending on the order by which they are placed on the update statement it resolves 2 different error messages:

• $setOnInsert before $inc

  db.t.update({_id:'12345',hour:12345},{ $setOnInsert: { rpm: [10, 15] }, $inc: { count: 1, 'rpm.$0': 10, 'rpm.$1': 15 }}, { upsert:true })
  WriteResult({
  	"nMatched" : 0,
  	"nUpserted" : 0,
  	"nModified" : 0,
  	"writeError" : {
  		"code" : 16836,
  		"errmsg" : "cannot use the part (rpm of rpm.$0) to traverse the element ({rpm: [ 10.0, 15.0 ]})"
  	}
  })
  

• $seOnInsert after $inc

  db.t.update({_id:'12345',hour:12345},{ $inc: { count: 1, 'rpm.$0': 10, 'rpm.$1': 15 }, $setOnInsert: { rpm: [10, 15] }}, { upsert:true })
  WriteResult({
  	"nMatched" : 0,
  	"nUpserted" : 0,
  	"nModified" : 0,
  	"writeError" : {
  		"code" : 16836,
  		"errmsg" : "Cannot update 'rpm.$0' and 'rpm' at the same time"
  	}
  })
  

In either situation this operation should be valid since it's very common, specially for time series operations/use cases, where we need to preallocate arrays and increment counters if those already exist.

The code reveals that we do not treat this valid situation:
https://github.com/mongodb/mongo/blob/master/src/mongo/db/ops/path_support.cpp#L110

https://github.com/mongodb/mongo/blob/master/src/mongo/db/ops/update_driver.cpp#L270-L278