You'd be surprised how easy it is for that to go wrong. For example, an anaesthetician asked permission to photograph the process of administering to a woman going through labour for a textbook. She consented, photographs were taken (not even showing anything lewd), forms were signed and all was well.

Then her lawyer write back a few weeks later accusing the doctor of obtaining her signature under duress, so he said "ok, she doesn't want to be in the textbook" and deleted the photographs.

Lawyer smells blood in the water and takes the whole thing to court. Judge berates the doctor for deleting the photographs since they were potential evidence and rules that her signature was invalid since a patient in serious pain cannot consent... which is obviously a problem since that's kind of necessary to a lot of modern medicine.

Anyhow, she ends back in the same hospital a few years later popping out another kid and this time no drugs for her! Anaesthetician wouldn't do it because a judge had previously ruled she couldn't consent.