Mine is, missing out on an edition, due to being skint at the time/not knowing about the edition/finding out too late/preorders getting messed up/having to cancel preorders due to money problems (the list goes on...), and then not being able to get it due to people blatantly only wanting a profit off of it
On the other hand, my favourite part is the sense of pride when you look at your collection, knowing that not many other people have some of the editions you own xx