What kind of problems have you encountered on the trails that could have been easily avoided but which taught you a valuable lesson about what to do (or not to do) next time. If only you had brought a certain tool, spare tube,pump, water pack, tie wrap, duct tape etc. I find it valuable to learn through other people's mistakes (as well as my own sordid history).
My latest example: When patching a tire check the inside of the tire one last time just before you put it back on. Result of not doing so: Another flat 30 seconds later because of something that got in while carelessy putting the tire back on.