In my second month trading, I remember I put the gold futures on my Anvil LII and sat there watching how GLD moved with the last-trade of the gold front month futures contract, I was counting in my head the difference in the price and when it got really far away, my theory was that it would revert quickly - it looked like GLD was a little slow, even, then I remember seeing a HUGE divergence and I bought 300 shares (I was still trading 100 shares back then, a total newbie) of GLD... lol, what happened was the gold futures contract I was watching was pit traded and the pit closed, so the price stopped changing - I nonetheless thought there was some absurd huge arbitrage opportunity. Somehow I made a few bucks on the trade anyway.