If vid_widescreen is on and you're in windowed mode, you can resize the window in such a way that the ammo counter bleeds over to the opposite side of the screen.
Also the vid_fullscreen CVAR seems to just not work. If you try to change it, literally nothing happens.
Also, if you change the vid_borderlesswindow CVAR while in fullscreen mode, the alt+enter shortcut stops working as well, until you restart Doom Retro.