1. Overexertion, whether it was years of overdoing it in a general way because everyone was encouraging me to do more (I spent eight years trying to do my undergrad), or my occasional attempts at a graded exercise regime where I always get over-enthusiastic and exercise myself into a relapse.
2. Hormonal contraception. The first time was Depo Provera, which I was told was my only reliable option to prevent pregnancy and would have no effects on the ME. I got most of the symptoms of the menopause at the age of 22 and a hellish ME deterioration. Years later, I was put on Cerazette as a supposed treatment for PMDD by a gynae who was ignoring all the evidence that progestogens make PMDD worse, not to mention my dire record on progestogen. Naturally, it made both the PMDD and the ME worse, at which point the gynae decided that I didn't have PMDD because I hadn't got on with her pet drug. (She had never actually bothered to do a proper diagnosis involving a symptom diary.)
3. Some of the widely-recommended methods for dealing with circadian rhythm disorders, such as staying up all night or going to bed three hours later every day. Messed up my sleep even further and had a pretty dire knock-on effect on the ME. I did eventually find treatments for the circadian rhythm problems, which I've described in the "3 things that worked" thread.
I do badly on a lot of medication, but I'm not really counting temporary side effects here (e.g. feeling emotionally disconnected on Prozac, getting stomach pain from NSAIDs), just things that triggered an actual relapse.