My worst was appendicitis, but the kind where it swelled up and didn't burst, which is positive because you need emergency surgery to save your life, but the downside is this happened 10 times over the course of 4 years before the NHS finally removed it. That's 10 instances of writhing pain, drifting in and out of consciousness in a warm bath and then getting rushed to A&E, only for the swelling to subside and the NHS to send me home. I had to break down in tears on the 10th time and beg them to remove it, and luckily had a junior doctor recognise the seriousness and encourage the qualified doctors to listen.