I started out in 2010. My biggest overall mistake in investing was aimlessly buying and selling stocks, ETFs, ETNs mostly by reading analysis on Seeking alpha, motley fool , blogs and the like. I was moving in and out of stocks in a matter of weeks, months. I wasn't buying businesses, I was buying pieces of electronic paper, the exact opposite of what Graham and Buffett ask investors to treat stocks as.
Finally in 2011, frustrated with no performance I read some books like Intelligent Investor, Pat Dorsey's book on moats. Warren Buffet's letters to shareholders and I am beginning to appreciate the concepts of earning power, moats and ROIC.
In terms of mistakes I came close to losing permanent capital. I once put 15K in a chinese packaging company called BEST(Shiner international) without understanding anything about the business.