Can a tandem's front bottom bracket fail catastrophically? Many tandems have a front bottom bracket that looks like this: 



The front bottom bracket is mounted off-centre in an aluminum shell called "the eccentric."  When the pinch bolts shown in the picture are loosened, the eccentric can rotate within the bottom bracket shell to tighten or loosen the tandem's timing chain (that joins the front and rear cranks).

If the pinch bolts broke, or if the braze-ons they fit into sheered off, would the bottom bracket shell pop open?  It seems to me that the integrity of the bottom bracket shell is totally dependent on the bolts; that you'd bend the frame quite badly if they broke.
First off, in order for there to be any sort of a "crisis", both bolts would have to fail, an unlikely possibility on a reasonably well-maintained bike.

But, beyond that, the integrity of the frame doesn't depend on the BB shell resisting twisting loads.  Rather, due to the "truss" nature of the standard bike frame (tandem or not), the load on the shell is almost totally "radial" -- force into or away from the shell.  While this force would tend to "pry" the shell open, it's very unlikely to be substantial enough to cause catastrophic failure.