Some ancestors make life easy on us. They can be found in record after record. Their names are clearly written and spelled in a way that makes sense. They seem always to be recorded with enough extra information to really be sure that they are who we think they are.
Some ancestors seem to refuse to be identified. Everything is ambiguous. The name both common and questionable as written. The place is iffy. Nothing matches really well and several people might 
be the person you are after. Of course, it might be that none of them is right. It is time to try to paint that ancestor into a corner. Take what you think you know about the problem person. Born 1840? Try eliminating people who seem to have been born before 1835. Paint that part of the floor. Eliminate people born after 1845. Paint that part of the floor too. Next, what about places? Was your ancestor supposedly born in New York? Paint other states that aren’t neighboring. To be a bit more generous, maybe New Hampshire shouldn’t be painted since it is close and begins with “New.” If your source for New York was really questionable, maybe nothing at all in New England should be painted. The idea is to really put some thought into what the constraints are—what should really not be true of that ancestor. Try to eliminate people who fall within those unacceptable names, dates, places, etc. Who is left? Can more constraints be found, like occupation perhaps? Someone who was a machinist one year might be a machinist or a mechanic the next year but probably not a butcher.
If there are several people left in the unpainted corner, only more research can eliminate them. Is the last person, hemmed in by the paint the correct person? That still needs to be proven but from what you know, that person is the most likely candidate. Eliminate that person too and it is time to wipe away some of the paint and consider the people who were painted into those places.
Note: For anyone who is a glutton for punishment, the idea for this way of thinking about cornering our difficult ancestors occurred to me when I was reminded of learning a mathematical technique called linear programing. It might seem simple but it actually can get complex. (The version I learned in grade school was way over at the simple end of the spectrum!) I was amused to read that it wasn’t developed until World War II, when it was created for helping the Red Army optimize its planning, and was kept secret until 1947. Though painting our ancestors into corners isn’t quite the same thing as linear programing, that it is even very vaguely related to something that was a state secret within living memory seems pretty odd.