I know directly of one case in which one of the terms of the acquisition was that in case of the founder's death within the retention plan, his family wouldn't get the remaining part of the money (which would be given at the end of the retention plan only if he was still alive). This founder, rightly so, thought that it was an awful concept, since his kids needed to be protected exactly in the case of his death. The purchaser said they would have not changed the contract since that was a standard clause and that it was unconceivable to consider changing it. The startup called everything off the day before the deal (even after it was announced), because of this.

The clause that was impossible to make disappear suddenly disappeared overnight.

P.S. Congrats for Matasano, I am a fan of your company.