What have you learned the hard way?

My girlfriend and I wanted to check out the Tibet Butler Nature Preserve today since it is close to our house and reading bits about it online we found it has 4 miles of trails. Nice easy day hike before work later. Well when we got there we found out of the 4 miles of trails only the .8 mile trail was open. The rest were closed due to the rains recently. Not a problem since we didn't come that far but it got me thinking that what if we had traveled far to get there? Our 3rd hike ever a few weeks back was at Hal Scott and we were a good 5 miles from the parking area and came to a good size creek. We had not really planned to be walking through high water but didn't want to go back the way we came and see the same things again so we took off our shoes and went through the cold water. We had not packed a towel or anything to dry off since we did not plan to go through high water that day. Again nothing major but these little experiences are what we need to learn to avoid future annoyances. From this we learned to call the places we want to hike (if they have a number) to check if all the trails are open and the conditions. Also to have back up plans when we start doing longer travels to get to some other hikes around the state. We are just getting into hiking and learning everything we can from books and internet and such. Learning what items are best to pack (10 essentials) and what trails are good locally (south Orlando area). The point of this rambling is: We would like to know from more experienced hikers what were some lessons you learned the hard way or the best advice you've gotten that saved a ruined hike or day.