jmbl and greensleeves, I'm totally with you! When idly perusing just two trees, linked to different lines of my ancestry, I've seen:
child two years younger than his father
Same spouse, with same dates, attached to three generations(!) of the same male first name and according to the tree, producing over 20 progeny, many names and dates in more than one paternal generation(And if that isn't illegal, it ought to be!)
Long list of children born, less than a year between three of them, even overlapping with several of the same first name apparently surviving to further the next generations, the first brood ranging over 56 years of childbearing ( no wonder Mum seems to have vanished away to die unknown)
Individuals within said families born all over the British Isles, and elsewhere ( no, no seafarers on that line )
Same with census "evidence" - swapping age, spouse (and her age), job, location, children's names, sequence and ages! -And parents, of course, too!
Oh, and marrying MY ancestors off to the wrong family completely, banishing them abroad and foisting whole families of offspring onto them