this happened in my first u12, first gear was pushed right off the spindle.. can only be fixed by replacing the gearset, or the whole box. stop doing big launches and you'll be right..
as for reverse, has happened to a lot of people i know, but in other cars, basically learn to completely stop before you change from reverse to first or first to reverse, your new gearbox will love you for it.
also they're not at all cheap to fix, but mine was about 7-8yrs ago.