Migraines. Which, for reference, I've had severe enough to put me into hospital before. And have never had triggered in over 30 years of computer use, across thousands of apps and dozens of operating systems, by any other piece of software. So when I had a spike in migraine frequency because of Steam, I wouldn't have suspected it to be the problem without further prodding. I was LUCKY that I happened to trust Valve at the time enough to think it's worth commenting with feedback in their forums about the new UI which came along with the problem. A few weeks after making a post in their forums, I saw someone else mention that it's been causing migraines and triggering seizures for some people. If I hadn't gotten to a point of being ready to give up on finding out why there was such a spike in my migraines at the time, I probably still wouldn't have bothered looking into it, but I was frustrated and running out of options. My thought was "well it's probably not that but I might as well check to rule it out as a possibility" - except that when I did so, I ended up finding that the updated Steam client WAS the problem.
And since then, by spreading the word about the problem, I've found numerous others who have been similarly affected by the client, and got to be the reason many of them realised Steam has been affecting them. Because like I would have been if I hadn't happened to be in the right conversation at the right time, they never even suspected Steam until I mentioned it as a problem.