I think for small investors like me, one common mistake is “diversify”. Wait, isn’t “diversify” the first lesson we are taught to reduce risk? Yes, but as Peter Lynch said, “diversify” could mean “diworsify” too.
For example, I have 6 stocks in my portfolio now: CROX, HMIN, MR, GSIT, NINE and SBUX. Some are winners (CROX), some are losers (NINE). Had I hold all in CROX, my return would be much better. Of course you would say I had more risk too.