People underestimate complexity all the time. A drysuit, while not complex at first look, has a number of behaviours that are non-intuitive, like venting when you just start feeling light, not when you notice you're "going up" and adding air as you descend, because movement is extremely difficult and limited when shrink wrapped.
BC lift is another one, especially on this board, I see people all the time calculating their requirements and buying a wing that's exactly just as much as they need, without ever thinking they might need more in an emergency, or realizing that the stated lift capacity is often not really available.