Giraffes, being one of the tallest mammals in the world, have a failsafe called rete mirabile to prevent them from dying from excessive blood pressure while lowering their head.
Due to their long necks, a formidable amount of blood pressure is needed (quoted from animals.about.com):
A giraffe's heart has special adaptations to enable it to pump blood up the animal's long neck to its head. A giraffe's heart has the formidable task of pumping blood at high enough pressure so that it can flow up the giraffe's neck to the brain. To accomplish this, a giraffe's heart is specially adapted. It can weigh up to 10 kg (22 lb) and generates twice the blood pressure of other large mammals. Having enough blood pressure to pump blood to the brain when the giraffe's neck is extended upward is one challenge, but when the animal lowers its head it risks injury due to excessive blood pressure. To counter this, giraffes have a pressure-regulating system known as the rete mirabile which restricts the amount of blood that rushes towards the brain when the giraffe lowers its head.
How did this evolve? I believe in evolution but I can't wrap my head around this. If an "overload" of blood would reach the giraffe's brain while it is, for instance, drinking water and either instantly killing the animal or severely wounding the animals mental and physical capabilities by rupturing the brain, how could a failsafe like this evolve?