After updating to the latest core, games are no longer being output using the “Pixel Aspect” option by default, which means that MAME is applying scaling to the image before RetroArch. This results in very ugly artifacts when the screen scrolls. (and also the wrong aspect ratio, but that’s less of a concern)
[ul] [li]Latest MAME Core [/li][li]Old MAME Core (2016-03-10) [/li][/ul]