There are hundreds of millions (if not a billion) people who believe in Karma (Hinduism taking up the bulk of these numbers). The Hindu's have a very elaborate system describing Karma and how it works..as well as other faiths such as Buddhism.
Then there are the "lay-folk" who have a general superstitious feeling about it..and of the people I know who somewhat believe in it, they can all give multiple examples of things that have happened in their life to justify that belief.
It could just be co-incidence and the perception of correlation when in reality the events were completely independent of each other, or it could actually be a real thing. Not all of these people take into account ALL the things they do, so when they selectively choose two events and use Karma as a factor in both, but leave out a bunch of other stuff then it doesnt quite make all that sense. However, a lot of these events are usually on the exact same subject or take into account multiple decisions being made in the same sphere of events that led to some sort of concluding event. Ultimately, it is unverifiable.
I guess I believe in Karma a bit. Since I have no recollection of a past life or lives, and that is the basis for a lot of Karmic Systems, I do not fully believe it...but I have noticed enough in my life and the life of others to give it some credence. I do not go pointing out every good thing that happens as karmic justice for something bad that had happened, nor do I say that every bad thing that happens in karmic justice or something else bad that I had done, but the thought crosses my mind at times.
EDIT: I guess I should mention that I did a past life regression hypnosis and I got a name and vision of who i "was" in my past life but I have no idea and doubt its authenticity. I googled the name after, and it was an author in the 1800s that actually lived in my area (at the time I was writing a lot on my website, was offered a job as a beat reporter, had about 30 pages done of a philosophy/logic book and finished a childrens book). I was kinda freaked out tbh