I haven't been online for a long time; I had the latest version of MESS and MAME, 0.117.
In MESS 0.123, atvga (and 'not working' at586) are not going any further than the initializing screen, and exit with:
[pre]fatal: install_mem_handler called with ROM or RAM after initialization[/pre]
at, ibmat, at386 and at486 still start up, even if it is to a black screen on the 286es.
Also, I've found another (harmless) bug with the old UI with the file selector.
Go into the disk part to change files, select [empty slot], then hit Tab. You will end up in the file selector again rather than in-game, at the bottom, but in the File Specification (*.*) menu. Instead of changing the filter though, hitting Enter selects the file it's stuck on (in my case, it was ui.bdf in the mess directory, so it ended up trying to load ui.bdf as a 3.5 inch floppy disk!).
That bug can happen in any driver, also tested in sms and nes.
Numpad items cannot be used in the file specification menu either. Numpad 0-9, divide (slash), multiply (asterisk), minus (hyphen), plus and comma (decimal point/period) and Enter cannot be used. It's easier to hit *.* on the numpad than hitting [Shift+8], dot, [Shift+8].
Another thing is, if you start in windowed mode, go into full screen and change a disk/ROM image in the file selector, it resets but goes back into windowed mode rather than staying full screen. There's also a backslash or two being added when you change files, so newly changed files still don't load properly.
Sorry if any, or all, of this has already been listed, I only just signed up to Bugzilla a few minutes ago. (any admins can delete the Hotmail version of me from there, it was an accidental auto-complete and click on my end)