An extremely serious issue.  The problem is obviously worse in non-court venues because of detection devices and security at the courthouse which do not exist at the unsecured and sometimes isolated locations of mediations or depositions.  If you believe there is a potential for violence, consider changing your venue to a courthouse where metal detectors and security personnel will enhance safety.  It isn’t common, but such services can be brought into other venues.  Be alert and aware.  Communicate with your mediator and opposing counsel.