I think you missed the whole point that I was making about acid.

If you're handling a severely dangerous chemical, and have the proper precautions and methods in place to mitigate risk, and you do something stupid, it is you who messed up. You are assuming a greater risk, after making an informed decision.

But she ordered coffee that was as dangerous as acid. Again, her actions, or even carelessness, should have caused nothing more than some mild temporary discomfort, and for her to be out the pocket-change that is the cost of a McDonald's coffee. Instead, due to the extreme negligence of a corporation who was notified that their product was dangerous, and that people were being severely injured, continued to serve the product at dangerous temperatures (hot enough that one could be expected to get third degree, full tissue burns, after a couple of seconds of contact). In fact, I believe there was some testimony in depositions that said that one of the reasons behind their decision to make the coffee so hot was actually to prevent people from getting refills. If someone bought a coffee and a biscuit, the coffee would still be too hot to drink by the time they finished their breakfast so they would leave with the first cup instead of sitting and drinking several).

To restate. She didn't assume a risk to handle a material that could cause her to spend 8 days in a hospital and have skin grafts, she bought a food that is intended to be ingested (McDonald's own experts acknowledged that it could not be ingested without major injury at that temperature). And, the only way the situation is similar to you assuming a risk in a chemistry lab, is if the acid reacted in a way that couldn't be anticipated and exploded with such force that it tore your arms off. No matter how much you think you screwed up, under no circumstances should acid have done that.