There is a great deal of work being done by NATS with the airlines and other operators to mitigate against level busts.
It is a serious issue but at least, in CAS, one and most likely both aircraft should be ACAS equipped so hopefully worst case scenario should be avoided.
Unfortunately the biggest risk with CAS incursions is the fact than non-transponding or non-Mode C aircraft can be inside CAS and adjacent to much larger aircraft and there's no safety net whatsoever.
IO540, how on earth do you manage to bring ATC pay in to a post on level busts?
Use of SFL info from Mode S and ultimately electronic checking of cleared levels possibly via voice recognition and/or datalink if we ever go completely that way are amongst some of the proposed solutions for reducing level bust events (along with airspace and procedure changes).