Credit risk is based on total debt, and since credit cards are a revolving line of credit, the entire credit limit is considered debt, even if the balance is zero. That's why you can improve your credit score by closing out a credit card that you never use and that carries a zero balance (which we did in order to qualify for a mortgage several years ago). In other words, it's a sum, not an average; there's no division involved.
Setting aside the fact that there are much more reliable ways to detect a stopped heart than blood pressure: If you're taking a running average of BP over time, a low reading after a heart attack would just be a single data point, and you'd probably have to accumulate a bunch of them in order for it to register (particularly if you got a high reading just before the event). Even if you take a reading every five minutes, which is ridiculously frequent for BP, the patient will be long dead by the time you notice. You should be acting based on the last reading; again, no division involved.
I don't see why a freezer would be based on running averages rather than the most recent reading either. Thermostats generally work off of two thresholds: a higher one above which the compressor turns on, and a lower one below which it turns off. That smooths out any measurement noise without taking averages. Even if you do use averages for some reason, though, presumably it will start measuring at _some_ point and the compressor will turn on; I don't see why the average would stay at zero.
I'm not convinced that `1/0 = 0` is correct in any meaningful way, but I feel like any situation where it would cause bugs more critical than a UI issue probably points to a deeper design flaw. After all, if the alternative is to crash on a failed assertion, that's not necessarily preferable in a life-or-death situation.