Many (most?) of us top-rope self belay with a shunt on a regular basis and are pretty casual about it. Most people I know use the Petzl method (as per instructions with the shunt) - ie locker from belay loop directly into metal eye of the shunt plus some sort of elastic system over the shoulders to hold the locker up (this both reduces the fall distance and orientates the locker/shunt eye correctly). I always use double ropes through the shunt, never use a back up, take many falls on it and have never had a problem. However a few weeks ago, Gavin Peckham using the above configuration experienced a total failure: the shunt locked open as he sat back at the top of a climb. If he hadn't hit a ledge on the way down he would probably have died. Bruce Sobey was there and, being an engineer, investigated and came up with the brief accident report below. You can get further details from him at bsobey at telkomsa dot net.
Recently Gavin Peckham injured his achilles tendon in a fall at Wit Umfolozi after the shunt slipped. After a full investigatin we are positive that the cause was that he had connected a karabiner directly to the shunt (as per Petzl's instructions) instead of using a short sling threaded through the eye. If the karabiner moves up, it may jam in the eye, and a fall can side load the shunt, causing it to lock open. I have a full report with photos showing how this happens, which I will send to anyone who requests it.
The upshot is:
1. NEVER connect directly into a shunt with a karabiner! Use a short sling or prussic cord threaded through the eye of the shunt to connect to.
2. Make sure your connection is not long enough to hook on the top of the shunt. If you only have long slings, tie a knot in the end of one to make a short piece to thread through the shunt.
3. When passing an overhang it is a good idea to tie a knot in the rope below you in case the shunt slips.
4. Some people also clip two quick draws around the ropes in case the shunt breaks. One is clipped around both ropes, and one between the ropes. Although with a soft connection there is a lot less force on the eye, it seldom harms to take extra safety precautions, especially when climbing alone.