If this indeed was a lawyers' mistake, it was a doozy:

Times:

JPMorgan and Bear were prompted to renegotiate after shareholders began threatening to block the deal and it emerged that several "mistakes" were included in the original, hastily written contract, according to people involved in the talks.

One sentence was "inadvertently included," according to a person briefed on the talks, which requires JPMorgan to guarantee Bear's trades even if shareholders voted down the deal. That provision could allow Bear's shareholders to seek a higher bid while still forcing JPMorgan to honor its guarantee, these people said.

When the error was discovered, James Dimon, JPMorgan's chief executive, who was described by one participant as "apoplectic," began calling his lawyers at Wachtell, Lipton, Rosen & Katz to seek a way to have the sentence modified, these people said. Finger pointing over the mistakes in the contracts began as bankers blamed the lawyers and vice versa.

As a relevant aside, I've never understood why new associates with no finance background get sent to do "due diligence" and "document review" for complicated financial transactions. If they saw something was amiss, would they notice? I've been told that they are asked to look for very specific things, and that the bulk of due diligence is done by the finance people. I'd be curious to know what those who have experience with such things think.