D'Oh!
The most bum-clenchingly awful reply all mistake I've seen happened after the whole company received an email from operations reminding us of the new password policy - 12 characters, mixed case, special chars, etc., and NEVER write them down or share them.
This poor guy (let's call him Gary) meant to forward the following to his mate, but instead replied all: "How the hell do we remember these without writing them down".
This was swiftly followed by the email - Gary would like to recall ...
Which was followed about half an hour later by an email from HR, saying Gary no longer works for Enron (sorry), and has left the building.
Ouch!