Last, do no harm
I've come to feel that any belief, philosophy, or endeavor taken too seriously causes extreme harm. The most benevolent religious commitments taken to the extreme go completely off the rails, as do the most reasonable philosophies taken to their furthest logical conclusions. Even attempting to do no harm perfectly is likely to do immense harm, albeit in some unexpected way.
I'm self-conscious that it's taken me so long to see this, because it seems so obvious now. It only became clear after learning a bit about Nietzsche's criticism of philosophical stoicism, including that it leads adherents to act callously toward others and forego the most important challenges in life. I haven't noticed stoicism encouraging that kind of attitude, but when taken to its extreme, perhaps it does! That's one more reason for me to proceed with relative caution, rather than thoughtlessly adopt the worst of that worldview. Learning more about cults, in particular the Heaven's Gate UFO suicide cult, also helped me see this. Cult members, even those who commit terrible acts—especially those who commit terrible acts!—are not evil or stupid. In fact, they're often very intelligent and they almost always have the best intentions. They're just extremely committed.
If you find yourself in the 99th percentile of some endeavor, stop, slow down, and re-evaluate. A little common sense goes a long way, and at that high level of attainment, that's what you need most.