I failed the NAV section on my PPL when I did it. I had to divert to Fenlans which is a small field in amongst a number of others, and isnt the easiest airfield to find. My mistake wasnt so much I couldnt find it, but that I didnt apply logic in identifying built up apear properly. I mistook one large town for another.
Piperarcher, Fenland is a very harsh target for a nav diversion! I flew in there over the summer. Found it okay (it's a lovely place) but when I departed I took off and turned to set course over the field, and realised I couldn't see it
... It took me a good few orbits before I was able to locate it again. Really difficult to spot as it's smaller than the surrounding fields, and doesn't have many resident aircraft.