I was monitoring the race on ultrasignup - it was 20% full, 22%, then 26% for a long time. I had already signed up for a couple of "warm-up" runs (a marathon and a 50-miler) but didn't yet feel compelled to sign up for the 100 miles as well. Mostly because I wanted to push the payment out to a later credit card cycle. Maybe I was still undecided - did I really want to do this? There was still a lot of time - to make up my mind, to train, to get ready, to pay.
I didn't want to miss it either, because it is a nice flat race with an indulgent cut-off time. Over a few weeks I checked it pretty often. It got to be 33% full, which meant 66 runners out of the 200 allowed had already signed up. Still no reason to hurry.
Then one day I didn't see my race on the site anymore. It was simply not on the list. I scanned the whole thing once more, clicked on this or the other link, couldn't find it. I imagined 134 runners signing up all at once. I thought for a few moments it was sold out.
Given my ambivalence I expected to be relieved. Hey, I do not need to do this race, it's sold out. This one was mine, easy, I don't want another one. Never mind.
Instead I was disappointed. I wished I would have simply signed up.
Later I found my race on a different list, of races about to open for registration in a few days. I didn't understand how it could have been more than 30% full if it was not open for registration yet, but I didn't fret about it. On the day it opened I promptly signed up.