Mr. Lee,
<dislaimer>I am not a programmer by trade</disclaimer>

The biggest mistake(s) I have made are ones that seem to be made by all new coders.

• Not caring if it is the right way, only caring if it works
• Not caring if it is maintainable as I will never need it again
• Not listening to the advice being given by more experienced coders and getting frustrated that they are not answering my question(s)
• Getting stuck pursuing a solution that is wrong for the problem because it originally looked right and I had already invested a great deal of time in it
• Not bothering to RTFM and just changing things randomly hoping they would work
• Being so stubborn about just solving the problem that I miss all the tangental information I come across researching it that would have helped me 5 minutes down the road with the next problem
• etc, etc, etc

I am still learning and I am still making some of these mistakes. It seems that it is only after a new coder is willing to stop making these mistakes that they are no longer considered "new".