It's impossible not to run into places you regret you never hunted at and then they were torn down or not pursuing a place you thought was haunted but couldn't get proof of it the night you were there.
For me, as a child living in a haunted estate in Northern Virginia and having a mother who was an historian who felt compelled to go to every historic site, the regrets often times revolve around these places. I have been a psychic my entire life, as far back as I can recall, and these sites my mother made us traipse around on summer vacations provoked a great deal of reactions from me. Having a child's lack of interest in what a place was named or where it was located, I have lost the ability to discern which buildings I would want to go back to as an adult and paranormal investigator.
There are also some sites that I greatly regret not getting back to, such as the Newpoint-Comfort Lighthouse or my childhood home where my deceased family members vowed to haunt. One recent site I discovered that was untapped was an amazing find but I didn't have the equipment or the team with me, so that one is on my "must-do" list.
I had a list of great finds in the Appalachia area, every one of them having been torn down in recent years.
Time is wasting and I have found that it's best to not have regrets when sites might be gone tomorrow. As someone who does urban exploration and photography of abandoned sites, I know that some that Julie and I covered in our Abandoned Places: Abandoned Memories (Desert Edition) book have since been torn down or succumbed to the elements. We were able to document their one-time existence. The same goes for ghosts.
So, no regrets hunters - Carpe Diem!