Just came across this again, and it rings so true!
I found a marriage of my relative Mary Ann Campbell to Richard Silvester, complete with a Silvester 'young lady' (I quote!) with her grandfather George Campbell in 1881. I found Richard and Mary Ann in the same census with several children, and busily followed them down the censuses, put them on my tree, my Lost Cousins page, etc. I wilfully overlooked the woman called Fountain who was down as mother-in-law because she didn't fit. I expect you know how this is working out.
Yup, A DIFFERENT Richard Silvester married a Mary Ann Fountain, a couple of years after the other couple. I've seen the marriage certs and the fathers are entirely different in name and occupation. (The 2 wives are approximately the same age, and both born in London, different areas but I am very shaky on London geography and what area might be decribed as what.)
'My' couple seem to have disappeared by 1881- my current theory is that they both died and the daughter was not visiting her grandpa but living with him.
SO, all that effort for no gain, except to re-teach me a useful lesson: same name doesn't mean same person.