3 in my lifetime that stand out:
when I was in high school, I bagged groceries at Safeway between X-C and track season. When the produce manager heard that I was a 'top miler' in the area, he said that he was a top miler in high school, too, back in the day. I figured a few questions could quickly determine his credibility; I asked him what his best time was; he said 4:17....impressive and believable so then I asked him what he came by the 1/2 in, when he ran that time; he said 1:47.....BUSTED!
A guy that worked at a gym I used to go to, (Sports Connection), had the whole place convinced that he had already qualified for the Olympic team in the marathon. This was 6 months before the trials!
A Columbian guy who was a legit 225 marathoner and 31 minute 10k guy would get his way paid to big meets around the US back in the 80's, as he was passing himself off as a 2:12 marathoner. I would see him in magazines with a low bib #, standing on the front row at NY, Boston, and other big road races.
Those are my 3 'characters'