I learned not to underestimate a deer's eyes. I had been told previously that deer had fairly poor vision and that their nose was what you had to look out for. This led me to set up a couple hunts in places where the cover was not so good, and I thought that if I kept still, that I would have no problem. This led to deer skylining me almost instantly and I lost out on a couple nice opportunities. I plan to avoid this mistake as much as possible in the future.
I also learned and am continuing to learn that the moment when you least expect to see a deer, is precisely when they seem to show up...so don't let your guard down.