My first stop would be Dundee, February 1869 where I'd gatecrash the wedding of my ggg-grandparents John MacGregor and Mary C. Barnes.
I'd ask John:
Why are you revising your age downwards by 6 yrs in your marriage certificate? Where did you study for that Doctorate in Medicine which you profess to have, (because I can find no record of it)?
Have you ever actually practised medicine? Why, at (really) 38 years old, have you never appeared in any medical directory? And where were you in 1841, 1851 & 1861? In fact, where were you born?
Why are you about to leave Scotland for ever, taking your new bride within weeks to the Wild West - Dakota Territory - where gold has not yet been discovered and it's all about gunslinging in saloon bars?
Why in later life will you start using the middle initial "L", and what does it stand for?
I'd ask Mary:
What does your middle initial "C." stand for? Is it a family surname? Why do you insist on abbreviating it in every document over a lifetime?
I'd ask Mary's father Robert (who was a witness):
Where were you born - at least decide once and for all whether it was Scotland or Ireland! You might think it is funny to alternate it in every census but I do not...

Hmmm...methinks I might not be a welcome guest at the wedding

Anna