Well, I can say that the most effective financial lesson that I ever learned is "know the difference between wants and needs".

I can say this is what most novice level of mistake that one can make. And the funny thing is most of us are not putting much thought before spending.  There are some items that may seem necessary at that time but after you bought it you feel like you have made a mistake. So before making any purchase just have a debate with yourself. Know the answer whether it is something that you want or something that you need.

Take an example of the treadmill that my friend bought recently he spent 11000INR on that high-end piece of machine. When, I asked him why? I responded with "I need this, I barely do any exercise" and the funny thing is he has a big soccer field size park at a walk of 5mins from home where he can do the exercise. That is not the funny part, The funniest of all is he could have bought a hanger for hanging his clothes dry, for those clothes are now on the treadmill.

I am not criticising anyone here. But we all make the decision of buying stuff without much thought. Most of us are driven by what we want, That we often forget to ask ourselves "Do we need it?"

regards
@inuke