Look at the source and ask is it a primary source, that is something created at the time it happened. Usually the primary source should be preferred. However one has to bear in mind that people don't always tell the truth when, say registering a birth or getting married!
If we are talking about the UK then as ij gonya says census returns aren't always accurate. They are not a primary source (apart from the returns we see from 1911.) The enumerator extracted information off the returns and entered them into his summary book which is what we see, lots of room for error (apart from institutions, but they had their own problems regarding accuracy!).
Lots of room for error when people were getting married or registering a death given that there was a high rate of illiteracy and much of the information was given orally to the Registrar or clergymen and there was not always consistency as to how names were spelt
It isn't always a case of deciding that the most records giving a certain piece of information must be right as quite a few people went to some lengths to hide the true facts about themselves.
In short you need to weigh up all the evidence and decide why perhaps a particular entry goes against the general flow.