It's what you didn't, couldn't, or wouldn't anticipate that really bites you in the back
As engineers, we engage many laws: Ohm’s law, Kirchhoff’s law, and the law of gravity, to name just a few. But there’s another less rigorous law that I find also applies to many engineering designs and decisions, and that’s the law of unintended consequences (LoUiC), along with its closed corollary, the law of unforeseen consequences. Although we can model, simulate, extrapolate, envision, and even speculate on the effects of a design, there are many harder-to-grasp real-world, regulatory, and human factors which affect the actual impact.
Consider, for example, the classic Easy-Bake child’s oven from Hasbro. This product has been around since 1963 (here and here), and uses a standard-base, 100-watt incandescent bulb as its source of heat. It’s simple, cost-effective, reliable, and very replaceable: what more could you want in a central element of a product’s design?
But developments far outside the toy world have intruded onto this venerable design. New regulations going into effect, restricting the availability of low-efficiency light sources (meaning incandescent bulbs) as part of energy-saving mandates, mean that the oven’s heat source may not be available. So Hasbro redesigned the oven to use a built-in heating coil, see here and here.
Ironically, there is probably no net energy saving due to this redesign. Yes, the derided light bulb is only about 10% efficient at generating illumination, but that means that 90% of the input power is converted to heat—which is what you really want in this application. So it is actually a pretty efficient at meeting the product’s requirement.
If you add in the cost of the redesign’s materials to accommodate the new heating element (there are safety issues, of course), I’ll bet the new design is no better, or possibly worse, that the old design. Further, if the new heating element burns out, the replacement module (if one is available, it’s not clear to me from the web page) will add to materials waste—or perhaps the entire oven will be tossed out (it retails for around $30); just think of the environmental impact of that.
I like to keep the law of unintended consequences in mind whenever I say to myself (or hear someone confidently proclaim) “that change is no big deal” or “if we do this, then clearly such-and-such will be the result.” The LoUiC reminds me that our ability to fully gauge the effect of our actions, however well-intended, are often not as clear-cut or foreseeable as we’d like to think—and sometimes are even the opposite. A little more humility and a little less confidence are always good. (Just envision what’s going to happen when people try to replace the incandescents used for illumination in freezers or regular ovens with those CCFLs or LEDs.)
Have you ever been in a situation where a seemingly straightforward design decision had unpleasant, detrimental, or even a contrary impact to what analysis and conventional wisdom “assured” would be the case? ?