Why can you sometimes get a dangerously high fever when you're ill? doesn't your body know it shouldn't make a fever that high that it might kill you?

Rare problem: Fever is part of normal immune response, improving the ability to fight certain infections. Even high fever isn't necessarily bad, and it is extremely rare for high fever from infection to be fatal. (Unlike fever from heat stroke.) Still, the immune system sometimes overreacts and contributes to severe problem, indluding death (e.g. from sepsis). We are not biologically and evolutionarily perfect.