Started up the Master System core for the first time in a while, and for some reason the screen was rotated 90 deg clockwise (so I would have to rotate my display counter-clockwise, which I don't think I've ever done).
Not sure if this is a bug related to a recent update or my settings, couldn't find a setting that fixed the issue so I had to reset everything to default to get the screen back to normal.
Anyway, probably not a serious issue, just thought it was strange.