I have noticed the same thing as you but I wouldnt say its the BEST lesson I have learned.
ITs really hard for me to choose which one is the best lesson I had. There have been so many. One good piece of advice is that never value too high your trips, if you are holding only ONE of them (say you have KT and board is Q 8 T T A) You can lose much, thinking you have superb cards, but there are many cases where your trips aint that good as you think. In given example, opponent may have full house or a straight and you can lose a loot of money, depending on seemingly good cards.