I'll start.
Funny enough, it was when I was trying to get rid of the last of my bcash from 2017. I kept waiting for it to go up again to dump, and it never did. I think I held until sometime in mid-to-late 2018 when it was a fraction of what I could have sold it for in late 2017. It made me realize then that long-term all of these altcoins are going to zero against Bitcoin and that I couldn't time the market very well. The last justification for altcoin ownership then died.