Okay, I'll share my experience with a memorable lesson from a terrible stock trade that happened to me:
I bought a 1000 shares stock called PNTC back in 2001-2002 for a little over $1 a share. I bought based on speculative news from a friend, and as the stock slowly fell he urged me to hold on. Within a few months it was worth less than .10 cents. I left for college and I held on with the hope that it would rise. Eventually, I gave up and sold it in 2005 for .11 cents a share (which it shot up from soon after - ARGH!)
The lesson: Do my OWN research! No matter how smart or knowledgeable a person may seem about a particular stock, I need to know understand the stock myself (or be willing to lose every cent). Also, I learned that losing money in stocks can be like quicksand. The longer I wait to react to a losing stock, the harder it is to get out. I find myself hoping and wishing instead of moving on to make money elsewhere. So, it helps me to know how much I am willing to risk, and discipline myself to get out of a trade quick if doesn't go the way I expected.
If I continue to learn from this experience, I think it will be an inexpensive and valuable lesson....

-Mavace