As I think back, I have left a lot of points/miles on the table during my life. Let me count the losses...

1) Sprint
I recall having a Sprint Long Distance affinity program in the nineties that had over 250,000 miles in it. Somehow, after a couple of moves that took place in quick succession and some laziness on my part, I lost those miles. Didn't exist anymore according to Sprint.

2) TWA
No explanation needed, but I lost a good 150,000 odd miles on that one and frankly saw it coming from the early 90's so should have known better. It was just that I believed Virgin was going to buy them out - silly me.

3) Citibank
Some years ago, I had a Citi card with a points program that morphed a couple of times. It eventually wound up being a program that awarded air tickets based on the distance traveled, one point equaling one mile of air travel.. So, if one wanted to fly DFW-ORD R/T, it would cost 1604 points. One year, Citi sent out replacement cards. I was living out of the country at the time and had not used this card for a number of months. When I returned to the U.S., I opened the envelope containing the Citibank card and called Citi to activate it. At that time I was told that since I had not activated the new card within 90 days of delivery and not made any charges on that account for more than 6 months, Citi had closed the account and I had forfeited my points.

3) Expired Delta miles:
A few thousand here and there lost due to expiration

4) Other Orphan miles:
Airlines I might have only flown once or twice. I consider these lost miles since I will never get to use them.

5) [added] United Mileage Plus miles:
In the mid-eighties, I flew United and BA quite a bit. BA miles were creditable toMileage Plus at the time. I had about 170,000 miles in MP and somehow never flew UA or BA for years, consequently losing those miles. Again, Laziness!