I may have posted about this subject before on The Lighter Side although cannot seen any recent threads in the past 2 or 3 years, but as we will all know, we will come across several brickwalls in our ancestry from time to time, some may be permanent such as gaps in PR's etc. Some may be demolished as more comes online or more trips the old fashioned way like looking through reams of census sheets/parish registers at the FRC or any record office (I did that 20 years ago or more).
But I do like a challenge. I have 3 direct ancestors who said "not born in county" in 1841 census (Essex, Oxfordshire and Middlesex), but the Grim Reaper claimed 2 of them in 1849 and Feb 1851 respectively, the last one died just 5 weeks prior to the 1851 census. If they had all lived to see the 1851 census which gave birthplace (often some just gave a county if living in a diff county, or even country if from abroad) then it would be too easy and I would know. The fact I am always digging for new info is the thrill of the chase.
I think the Essex resident not born in county in 1841 was born in Cambs or Suffolk. The Oxford one, not sure, maybe Bucks due to a lead, as he was a tin plate worker, and in 1851 (he was dead by then of course) about a third of the tin plate workers living in Oxford came from other counties such as Bucks, Berks, London, Bedfordshire, Gloucestershire, one from Norfolk and one from Cornwall. The London (Middlesex) one not born in county in 1841, well a long story as I cannot find her marriage to her first hubby, he was from Dorset originally.