I'm too new to the game (and too smart ) to have lost any miles worth mentioning in the thread.

My father lost more DL miles than I care to imagine when for two years in the mid 1990s he was flying LGW-ATL-TYS every three weeks in Business Class. He just never got round to using the miles and they expired unused

I've trotted this story out before but what the hell...The absolute worst, and one of the reasons why, even as not very regular traveller myself, I keep an eye on my FFPs is that for eight years my father was working regularly in Southern California. My brother, mother and I would fly out to see him in BA Club World anywhere from four to seven times per year. To credit miles to BA Executive Club you need to be 18yrs old but you could set up a Household account and anyone, any age could contribute. However my father never realised this so my brother and I lost equivalent to 100+ J trips worth of miles in the process. Plus some BA Gold wallet candy, not that I had a wallet at the age of 7 but....