I've noticed, when peeking at some online trees linked to ancestors of mine, that there are quite a few trees with HUGE lists of children for one or two couples. In some cases it's fairly obvious that someone has merged two - or in some cases three families together - Lazy research, the fathers all had the same name, and were more-or-less in the same parish, BUT the mothers' names on the parish records were not the same, and if you checked out carefully even only online the places where the couples lived were named, so in most of those cases it was even with elementary research possible to sort out which couple had which children, but they'd not bothered even to do that. - and the birthdates / baptisms for the children imply some fantastically short gestation periods, even if you try to work a system out where children were baptised at very odd ages!!! Do people not think? Or check? And, as already mentioned, these silly errors get copied, and even compounded....
-But - Wow! They've usually got 100,000 names on their tree, and ancestry tracing directly back to Noah!
They'll never change. And if you send them a correction, even if politely suggesting that your own researches have found something different... well, you're wasting your efforts.
