Re crashes - I didn't think of 2018 as a crash. Too small; I think of a crash as either being bigger (30-35% minimum), longer lasting or both. Fwiw, wikipedia claims that the common thought is that crashes are steep dive, bear markets last longer; arguably, from that viewpoint Nov-Dec '18 might be a crash, contrary to my instinct. That said, the quick dive did make my gut flutter, and panic is a key element of a crash. To me, the panic level at that time was narrow, as a previous poster suggested in other words, rather than the wide spread that would mark a Real Crash. So IMHO, Nov-Dec '18 was a hint of a crash, not the real thing.
Re 2008 - I adjusted twice. Beforehand, in 2007, I suspected something weird and crash-y was brewing, but couldn't figure out what to do. I had just gotten my first professional job, and I was guessing that cash might be the underrated asset of the day, so I mortgaged up on my house with a cashout that summer, which paid off some consumer debt that had paid for grad school groceries, plus gave me 61,000 cash. The 61k sat in the bank while I procrastinated and made payments on 16k of student loans. I knew I should probably invest in stock because the odds were better than anything else, but I'm lazy and hesitant. After some months, I paid off the student loans and began stock investing with 15k. Summer 2008, second 15k. Early Sept (?) '08, the last 15k.
We all know the bear market became a crash in Oct & Nov '08. I figured I didn't have any way to avoid that, so on with the plan. I avoided looking at balances except once every couple of months, though I read news. At one point, the 45k had become 27k though. In early '09, I started a quick series of $1000 investments using all my spare cash from the new job. At the time, my friend at work who liked stocks joined me in feeling that stocks were on sale, and wanting to invest more. My only regret is not automating the investments.
Now FIREd, but could have been quicker.