What should happen in the following case:

db.transaction(["foo"]).objectStore("foo").openCursor().onsuccess = function(e) {
  var cursor = e.result;
  if (!cursor)
    return;

  cursor.delete();
  cursor.update({ id: 1234, value: "Benny" });
}


This situation can of course arrive in more subtle ways:

os = db.transaction(["foo"]).objectStore("foo");
os.openCursor().onsuccess = function(e) {
  var cursor = e.result;
  if (!cursor)
    return;

  cursor.update({ id: 1234, value: "Benny" });
}
os.delete(1234);


As specified, IDBCursor.update behaves just like IDBObjectStore.put and just creates a new entry, but this might be somewhat unexpected behavior.