Running Linux Mint
Sometimes it does not happen but mostly as soon as I press alt+enter for fullscreen the alt gets stuck and then every time I press enter the screen mode changes.
Well, clean your keyboard?
It usually helps to just press and release alt again after you've gone fullscreen.
I don't mean physically...
It gets "stickey" as in it keeps being pressed whether I do it myself or not.
Could be a faulty PS2/USB connection or your key switches don't get released after you press them. I'd open the keyboard up and have a look at it (maybe even deep clean it). Otherwise, if it's not an expensive keyboard, just buy a new one.
Or look up if there is an issue of sdl with your distro and sticky meta keys
Probably a SDL issue, I saw last night it happens with the right alt key but not the left... No physically the alt key is fine it's just dosbox that it happens in.
Right alt can be special, dependibg on keyboard layout it can be seen as ctrl-alt for example.
I know there's been a problem in Ubuntu with Unity where at least one of the Alt keys may be used for the HUD. However, as of Ubuntu 14.04, even if this is not a problem, Alt can still not be used all the time.
It can, however, be used once the mouse cursor gets locked (which, by default, can be done using the Ctrl+F10 key-combo). If the mouse cursor is not locked then for some reason Alt key press events aren't sent, although key *release* events are (at least when I last investigated that).
