These days, it is a different story. Take this April, for example. It began with Anbang Insurance Group abruptly withdrawing its $14 billion offer for Starwood Hotels and Resorts, ceding it to Marriott International for a lower price. The next bombshell came three days later, when the Treasury Department issued new rules to curb cross-border mergers aimed at lowering an American company’s tax bill, a move that ultimately led to the breakup of Pfizer’s deal with Allergan.

Though the Treasury had long signaled it would clamp down on such combinations, many arbitrage funds did not expect that the new rules to be so cataclysmic. Before the rules were disclosed, the market gave about a 50 percent chance the deal would close.

Allergan’s stock dropped 20 percent in overnight trading after the Treasury announced its new rules, according to a letter after the deal collapsed from the hedge fund Ramius to its investors. The letter, which was reviewed by The New York Times, said, “We regret this setback, and ask for your patience as we follow our discipline to attempt to earn back lost value in a prudent fashion.” Ramius was already down 2.91 percent for March before the deal was scuttled, after being up 4.37 percent in February, the letter said.

But the pain did not end there.

Soon thereafter, even more mergers seemed doomed, especially because at least two companies have made overtures to kill transactions in cases of buyer’s remorse.

Energy Transfer Equity, a Dallas-based pipeline operator that initially had to coax a majority of the Williams Companies board to agree to its $38 billion offer in September, was frantically searching for a way out of the deal by springtime. Things started to look shaky when Williams Companies sued Energy Transfer and its chairman, Kelcy Warren, in mid-April, claiming he had breached the merger agreement. Then Mr. Warren said several times during the company’s earnings conference call last week that the deal “can’t close” because of a complicated tax opinion. Williams firmly believes it can.

Investors overwhelmingly think the deal, as it was agreed on seven months ago, is doomed. Of about 150 fund managers surveyed by Evercore ISI, 84 percent do not expect the deal to close in its current form.

Then there is the drug maker Abbott’s $5.8 billion acquisition of Alere, which makes medical diagnostics tests. At the end of April, less than three months after their deal was announced, Alere put out a statement saying that Abbott was trying to terminate their agreement but that Alere had denied the request.