Well, I am one who has tried to do trigger work (on a 1911 with spare hammers). And, yes, I have messed them up. I have also had some good ones.
In the 'old days' there were books on gunsmith and pistol smith work. The pistol books had good descriptions of how to work on sears to get smoother/lighter triggers. It was all about angles and making jigs to do the work accurately (and a very good stone). The book I used specifically described working on Colt SAA and 1911 hammers.
It is easy to see where someone might take a stone to their gun. Go to a shoot and hear everyone talk about how their pistols have been 'fixed' up. They try it, mess it up, then find out how expensive a replacement is. Put it up for sale.