Well, let's see...
1. Not paying attention to where yellow jackets were landing and taking off from on the forest floor.
2. Not paying attention to which plants had three dangerous leaves...
3. Not marking a waypoint on the GPS unit for the car before I got a mile deep in the woods (that one Sucked!)
4. Not noticing that there was a 400-foot elevation change on Google Earth between where I parked and where the site was (Ohhh, my aching calves!)
5. Not making sure the coordinates were in the right format before entering them in the GPS unit (DD MM.mm versus DD.dddd versus DD MM SS.ss ) :P
6A. Not writing EVERYTHING down that every old timer has told me.
6B. (Expecting my memory to be flawless after a decade and a half of digging, and 1000 sites

)
7. Not taking the adequate amount of gear on a long detecting trip.
8. Extra batteries? Who needs those??
9. Shovel? Who needs that?
10. Relying on permission that was granted a long time ago to gain access to a site that I had no idea had since been sold.
11. Not checking in with the property owner to let them know each and every time I detect on their land. (It builds a good relationship based on respect for them and their property. It also avoids HAIRY situations--as I only had to find out once...

) The secret to not pestering the heck out of them is to have enough good sites going at all times that you don't have to hunt in one spot four days in a row. (Also a good thing, since some owners will start to wonder what you're doing there all the time--and wonder if you're finding riches beyond measure

).
12. Stepping in dog poo, stepping on a snake, almost stepping down a well, allowing an electric fence to slip when I was holding it down with my shovel in order to step across it (WHILE I had one foot on either side of it!).
13. Finding my first IH when I was 13 and too excited to recheck the hole--then later noticed the shape of a Dime outlined against the IH where there was no tarnish. Could've been 10 pre-1900 coins in that hole, and I was only thinking about my first IH. Different owner for the property now--and no permission to rehunt it.
All Lessons Learned.
16 years of learning--and every time I try a new type of hunting, more learning. And still I forget to do something sometimes.
-Buck