Some shareholders have taken Dell Inc.'s slogan—"The Power to Do More"—to heart. They want the computer maker to do more than just accept a $24.4 billion bid for the computer group by its founder Michael Dell and private-equity firms.

The activist investor Carl Icahn is leading the pack of critics. Last week, he wrote to the special committee of independent Dell directors charged with evaluating offers, threatening to fire them and sue them if they don't back down on the deal and return cash to investors instead. The committee members replied that they are working hard for all shareholders.

Corporate directors, be afraid. Be very afraid. As financial and economic conditions improve and mergers and acquisitions pick up, your acumen in judging deals and upholding the interests of shareholders will come under intense public and legal scrutiny.

The Dell drama is still unfolding, but for a cautionary tale of how boards, even when they may be well-intentioned, can harm investors of a takeover target, take Gleacher & Co. Shares in the small investment bank have lost more than 60% in the past year as the prospects for a deal evaporated, business dwindled and star traders left.

Ironically for a firm that bears the name of Eric Gleacher, who made his name advising on big deals in the 1980s, the company failed to sell itself. At least as some critics see it, its independent directors are to blame.

Here is what happened. On June 4 last year, Mr. Gleacher, then chairman, received a letter from Ronald Kruszewski chief executive of Stifel Financial Corp, a rival firm. Stifel was interested in buying Gleacher for $250 million, according to the letter, which was reviewed by The Wall Street Journal. Excluding a special bonus for top employees, that price was some 80% higher than Gleacher's shares on the day. My intelligence is that both Mr. Gleacher, who owns around 12% of the company, and MatlinPatterson Global Advisers LLC, a private-equity firm that owns a further 28%, wanted to entertain the proposal.

Gleacher's board appointed a special committee and launched a "strategic review" to determine whether the discussed bid was in the interests of all shareholders. The committee in turn hired Credit Suisse AG to check for alternatives.

Months passed without a decision, and Gleacher's shares continued to slide. In September, Mr. Kruszewski wrote again, this time to Thomas Hughes, Gleacher's chief executive, with a $150 million price tag.

In February, Mr. Hughes told investors that the review had ended without finding an offer that "adequately reflected Gleacher's value." Ten days later, in a highly unusual move, the four directors on the special committee—Robert Yingling, Henry Bienen, Robert Gerard and Bruce Rohde—said they would leave at the annual meeting in May because MatlinPatterson and Mr. Gleacher, who resigned the chairmanship in January, would have opposed their re-election.

Behind the scenes, the directors, Stifel, Mr. Gleacher and MatlinPatterson disagree on some of the key events. Did Stifel offer a good price, since it was pitching the bid well below some metrics of Gleacher's worth? Was the Stifel bid real, or just an attempt to kick the tires and maybe poach some employees? Was the coolness by directors and management toward the letters motivated by the desire to keep their jobs?

One number, though, is incontrovertible: Gleacher's shares have lost nearly 40% since Stifel's first letter.

Like its Dell counterpart, Gleacher's special committee deserves some sympathy. They could have been damned if they sold the company for less than its worth, and now they are damned (by some) for missing out on a potential offer. But unlike at the computer company, where Mr. Dell's dual roles as bidder and CEO give him an incentive, and some leverage, to push down the takeover price, shareholders controlling nearly 40% of Gleacher wanted to sell.

The special committee and Credit Suisse should have been supersure that better options were available before declaring that they couldn't find a decent offer. And Dell's shareholders will get to vote on the deal, while Gleacher's never did.

The bank's fate underlines the dangers of taking a wrong turn during a deal. Takeovers are a zero-sum game: For someone to win, someone else has to lose, unless you believe in the mumbo-jumbo about "merger of equals." As such, M&A talks are often riven by conflicts, diffidence and self-interest.

Independent directors are shareholders' last line of defense against a bad deal, or striking out on a good one. Unless they take that job seriously—and are held accountable by shareholders—the much-predicted M&A tide won't lift many investors' boats.

—Liz Moyer contributed to this article.

—Francesco Guerrera is The Wall Street Journal's Financial Editor. Write to him at: currentaccount@wsj.com and follow him on Twitter: @guerreraf72.

A version of this article appeared March 12, 2013, on page C1 in the U.S. edition of The Wall Street Journal, with the headline: Failed Bank Deal Provides Warning.