I have come to a dead end with one ancestor in the early 1800s.
There is no record of either his birth or his death to be found, just his marriage and the baptisms of the children, but his wife was definitely a widow before 1850, and almost certainly before 1840 when she was 75 and living with one of her widowed sons.
Someone else's tree gives a date of death for him that would have made him over 100 years old (which we do know he wasn't). They have failed to notice or take into account that the ancestor in question had a son with the same Christian name who died in the year they have used.
They have also added some parents for our ancestor, seemingly on the grounds that someone with the same name was baptised in the same large county around a date that would have been feasible. They haven't checked any further to find the records showing that it can't be the same person.
I have found dozens of similarly 'lazy' errors relating to other people on my tree, but am slightly afraid of contacting the trees owners in case they take offence, though I will be grateful if anyone contacts me to point out my own errors. I know there will be some, as I occasionally spot something myself that I have overlooked or recorded wongly by mistake.
It is a fascinating hobby, though. Sometimes establishing a link requires a long jigsaw-type process, and a bit of luck, but it feels great when it comes together.