The following code compiles, but it doesn't set change object because it is const:

const QJsonObject object;
object["foo"] = "bar";

It doesn't look too harmful in the example above, but if there are just a few statements between the two statements in the example it becomes a nasty trap.

In Qt-6, QJsonObject::operator[](const QString &) and QJsonArray::operator[](int) should return a const-ref object to eliminate this trap.