On QLineEdit if you press backspace too many times, the first character you type is painted and then replaced with the second character. Construct the QLineEdit like this

    QLineEdit* edt1 = new QLineEdit(centralWidget);
    edt1->setInputMethodHints(Qt::ImhNoPredictiveText);

The input method hint is the key here to get the bug. And second note is that you press the backspace at least one extra. See the screenshots. Probably the extra backspace moves the text selection pointer too far to the left and this leads to painting.