When using QSettings to store the state of a QMainWindow, the toolbars might not be restored correctly if some actions are hidden.
For every time the application is run, the leftmost toolbars shrink gradually until they are no longer visible.
Resizing the toolbars works in some cases, sometimes it gives an assert, but if the app is run to many times, then the toolbars can no longer be resized as the handle is invisible.
Resizing the mainwindow does not make a difference.

There is also a case reported where this happens even when the actions are not hidden, so it could be caused by other factors.