We always learn from our past. Is there any particular mistake that you've made comes to your mind which costed you big time?
Mine would be.. Lack of security. I had a good authority site which had a few high ranking pages. I'd put all my eggs in one basket and gone all in with the site, but it got hacked and hacker stuffed awful spammy links in my pages which only the Google spider could see. I found it one day looking at it from Google's cache, spammy links were all over the site. My site rankings had taken a big tumble and I had no idea why but this explained it. I'm not sure how exactly it got compromised but it might've been a keylogger that gave the hacker access to my FTP or maybe it was a server leak.