[JshWright]

It didn't 'slip by', it likely happened very quickly, without warning, and the doctors and nurses certainly did everything they could to save him (I don't know anything about the specifics of this case, but I'm familiar with the general practices).

There are various medications used for sedation that you wouldn't be exposed to in the course of 'normal' life, so the first time you're exposed to them is on the operating table. That makes it very hard to predict things like this.

It's also possible it wasn't actually an allergic reaction, but one of a handful of other life-threatening reactions that can take place during medical procedures (e.g. https://en.wikipedia.org/wiki/Malignant_hyperthermia )

No procedure is without risk. It is important to weigh benefits of the procedure against the risks (your doctor should be able to help you judge your specific risk factors and explain the benefits).