There's shorts and then there's shorts - As Stek relayed from Red's Battery Clinic a few posts back, what happens with an internal cell short (one cell, not the whole pack) is that something fails in the insulating barrier at a confined point and it tends to blow itself out. Bad for the cell in that it isn't going to function right anymore, but it's very unlikely to get excessively hot before the shorted path opens up by itself. That scenario is harmless in a parallel pack config, since it's going to only draw an insignificant amount of current - it's OPEN after the initial failure.

Then there's the barn burner variety of battery PACK short like Raven had, which can happen because of failed insulation of external wiring. Could be wire-wire, wire-cell case, improperly secured battery tabs etc. Some packs aren't made very well and are more susceptible to this kind of failure. It's quite possibly going to go up in flames. How to prevent that? Buy a well made pack. Protect it and all wiring from chafing. Route wiring safely away from everything else that could pinch it, rub it etc. Use a softer material than the insulation is made of to retian the wiring, something like the little triangle holders that JTEC makes are a good example. Take battery packs out of service if they have been involved a crash.