Whenever Mt Gox coins get moved even though there is no proof of them actually dumping it on the market, the entire market is thrown into panic mode. And it's not just Mt Gox - all sorts of news, relevant or not, get people into panic selling mode.

Obviously, that's not what you want. Because selling at low prices is the exact opposite you want to do.

This dump is probably not triggered by the Mt Gox rumors alone, though. The $10k resistance proved to but too much for BTC to go through, but the same principle applies - don't panic sell in a dump, it's just not a good strategy.