Even the most brilliant minds in finance can make terrible mistakes.

And when they do, the results are usually as bad as a lame summer blockbuster that ultimately flops.

1. They don't know when to quit

Is there anyone who believes that MF Global (MF) will make a comeback? The company didn't suddenly collapse; it has been going downhill for quite sometime. MF Global filed for bankruptcy Monday.

While there were signs as recent as Friday that the firm had bought itself a little more time, the fact is that the company has been declining steadily since August. From Aug. 1 to Oct. 21, MF Global shares lost more than half of their value. But investors hung on… only to watch the stock lose another 50% on Oct. 25.

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As of Aug. 1, MF Global was trading at $7.37; as of Friday, the stock was halted at $1.20.

Who were the investors who kept holding on? No one knows. But with takeover rumors and other semi-positive reports, many investors were hoping to cash in. However, this is not one of the scenarios in which they were able to pull that off.

While it is wholly possible that MF Global could emerge from bankruptcy and become a strong company once more, it could take months -- or years -- for that to occur. But some investors are holding on anyway, and others are buying in.

2. They support bad leaders… until it's too late

For all his accomplishments at Disney (DIS), Michael Eisner also reportedly caused a lot of trouble. He attempted to pass on "Lost" and "Desperate Housewives," both of which proved to be very profitable for Disney's national television network, ABC.

He also reportedly passed on American Idol, which made the News Corp.-owned Fox (NWSA) a ridiculous amount of money. These are just a few of his blunders, and yet he was not ousted until early 2005.

Carol Bartz, the ranting corporate exec who used to run Yahoo (YHOO), was supposedly a disaster from the day she was hired. But the board kept her on anyway. While some expressed their disgust for Yahoo's corporate team, many investors sat quietly, allowing the company to continue to deteriorate. Hewlett-Packard (HPQ), of course, was in a similar mess this year.

Fortunately for Disney, the company was able to reshape ABC into a prime-time contender, though it is still second to CBS (CBS). Yahoo, on other hand, has yet to begin its road to recovery.

Bottom line: investors need to rally against bad chief executives and other corporate leaders. If that doesn't work, they should consider dumping the stock associated with the troublesome chief exec.

3. They open their mouths

Mom always said that if you can't say something nice, don't say anything at all. If only she had known to add, "And if you're about to say something stupid, stop talking immediately!"

Lloyd Blankfein, the Goldman Sachs' (GS) chief exec who lawyer-ed up recently, got himself into a bit of trouble with the media when he said that he was doing "God's work." The original story from the Times Online has mysteriously disappeared (this link now re-directs to thetimes.co.uk/tto/n​ews/), but the Huffington Post and other sites still offer snippets of the story.

Blankfein later tried to take back his remarks, telling reporters that it was simply a "joke." But this was no laughing matter. His critics didn't buy it then, and they surely don't buy it now.

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