I always check that they have been logged in recently before contacting anyone.
It's surprising how many contacts go into "denial mode" when you try to help someone along, which I usually do by providing information which they may not have had access to. I think this is the most tactful approach, trying to avoid the "you are wrong".
One contact a couple of months ago had their great grandmother, who married in Leicester to a Leicester family, being born in Coventry rather than Leicester. She had quite a common first and last name, but had Emma as a middle one. The groom had an unusual first name and a VERY unusual surname.
When I passed on the bride's proper parentage, and where to find the same details I had come up with, they insisted upon a Coventry birth. Yes there was a girl born there, but the registration has no middle name, while the Leicester one has Emma to match every other record of the woman.
I provided a transcription of the 1912 marriage entry from FindMyPast, told them where they could see the image, and mentioned that the bride's address matched the 1911 census entry, and her father matched my research rather than anything in Coventry.
They have decided that there must have been a second man with the exact same unusual names as their great grandfather, living in 1912 in the same road as where their ancestor lived in 1911, and with a matching age, trade and father's details. They have simply removed the 1912 marriage from their tree, presumably in the hope of tracking down a second marriage which the GRO have never heard of.
Well, I tried to help a 4th cousin, but there's no point in flogging a dead horse...