The sell-off was already in full swing in mid-January, but it has intensified in recent weeks. So, assuming that just because a stock was down a lot to kick off the year would have been a big mistake. Or assuming that just because Microsoft (NASDAQ: MSFT) is highly diversified doesn't mean it can't sell off in a major way.It's a common mistake to look at a beaten-down stock, especially one that is down big in a short period, and assume that it can't keep falling. Just look at a chart of the 10 largest holdi ...