I don't think that there is one particular worse case scenario, so I wouldn't attempt to guard against something so specific. High inflation, large market drops, or little to no (real) returns over an extended period, or some combination of the three could all be a SIGBTW^ scenario.
Regarding the Great Depression example, keeping a large percentage in bonds is good for a large market drop, but terrible for high inflation. So while guarding against one bad scenario, you're over-exposed to others.
^ Shit, I'm Going Back To Work ( I just made that up)