A JavaScript to show the possibility of clients not using client-side field level encryption may still be able to accidentally insert plain text data.

csfle-accidental-plaintext.js

// Insert another document from plainDB object with plain text.
// Remember the previous inserts were on csfleDB object (with Field-Level encryption options)
plainDB.getCollection("patients").insert({
  "_id": 3,
  "medRecNum": 3,
  "firstName": "Jason",
  "lastName": "Doe",
  "ssn": "333-33-3333",
  "mobile": "333-333-3333",
  "comment": "Jason Doe SSN/Phone should have been encrypted, but the app/dev forgot to do so."
});
plainDB.getCollection("patients").findOne({"_id": 3});
// You could see that data is stored in plain text because
// someone happened to insert plain text by mistake
/*
{
  "_id": 3,
  "medRecNum": 3,
  "firstName": "Jason",
  "lastName": "Doe",
  "ssn": "333-33-3333",
  "mobile": "333-333-3333",
  "comment": "James Bond SSN/Phone should have been encrypted, but the app/dev forgot to do so."
}
*/