qeventdispatcher_win.cpp defines this kind of user Window Messages:

enum {
    WM_QT_SOCKETNOTIFIER = WM_USER,
    WM_QT_SENDPOSTEDEVENTS = WM_USER + 1,
    WM_QT_ACTIVATENOTIFIERS = WM_USER + 2,
    SendPostedEventsWindowsTimerId = ~1u
};

Other applications may expect the same user level Window Messages which can lead to unexpected behaviour. This behaviour could be avoided by using RegisterWindowMessage method (https://msdn.microsoft.com/en-us/library/windows/desktop/ms644947(v=vs.85).aspx) which makes the message unique.