I prefer to do my own finding out and searching, but from time to time I'll browse specific individuals on other Ancestry trees, and several times I've found what seem to me to be glaring errors - my great grandmother assigned a different husband, and a few extra children ... and that tree built on from one of the "extras" - Sorry, no. Not that lady! A man mixed up with his cousin, my direct ancestor, with a very similar first name and the same surname, and of course the wrong family assigned, via, I thought at first, careless research, until I saw several other trees copying the same error.
I contacted the people with these trees, even pointing out how the two families had become confused, and separating out the confused lines for them, adding certificate details and a parallel run of census references in two cases that should have shown how a little more care in matching details would have made it obvious they were on the wrong branch ... but in most cases they happily continued sawing it off behind themselves, and the errors remained on show.
It compounds the innocent errors when people simply disregard through laziness the effort of revising their "own" research and findings.
On the two occasions people have kindly pointed out that I'd made an error, assigning in one case the wrong baptism, where the dates were very similar, and the parents' names the same, and in the other where I'd killed off a young member of a family in error for her cousin, I've been very grateful, and when I've checked it all out properly, amended my own records, with grateful thanks to the person who pointed it out.
-But it is as jillruss says, all part and parcel of everyday life now. I remember trying so hard to convince students years back that copying out slabs from internet sites did not, in my book, equate with personal and original research, but it still happens....