Only one really - Lagavulin 30yo that I saw a few years ago. At the time - it would have been the most expensive bottle I've ever bought (not now though - ouch!) so I hesitated. Then, it was gone. Over the past few years, they have come up once in a while - someone on this site was even kind enough to let me know of one in NYC I think last year - but I haven't sourced one out. Someday, I'm sure I'll grab one though...
Otherwise, there's always a "could have, should have" moment: whether it's PE release whatever, PC5s, Laphroaig 30s (definitely covered in that department!), old Ardbegs released in the late 1990s, or - even more generally - lots of common bottles that were much more reasonably priced before price rises of 2006 - 2008, there's always going to be something newer (or older) that you'll regret not buying. As it is now, I have enough whisky to probably last me the next 20 years, so I'm not stressing
