I think we all know that moment... A coworker inadvertently sends a "reply all" email instead of replying to one individual, and he/she doesn't realize it went to a large number of people. What's worse is that the message was clearly not meant to be shared broadly and was a snarky response about someone on the thread. What is the best way to deal with situation? Do you tell the coworker he/she has shared information broadly? Do you pretend it didn't happen? How should the sender remedy the situation? With another reply all?
Don't reply-all. I've been in e-mail threads of 100+ e-mails with people saying "Unsubcribe", "Please don't reply-all", "Please everyone stop replying all to the 60,000+ person company mailing list", etc....
Don't reply to them. They made a mistake, lots of people are correcting them on it. Probably by reply-all or sending them a message.
Do you pretend it didn't happen? How should the sender remedy the situation? With another reply all?
Ignore that it happened. The sender should ignore that it happened too.
I give these answers because: more communication on top of mass spam does not make the receivers happy, many people understand mistakes happen (a few wrong keystrokes or auto-complete gone wrong), most confidential material that may be contained in the Reply-All is probably going to people with a non-disclosure contract, and in general I don't care if I get e-mailed something I shouldn't as long as people don't keep reply-all'ing or e-mailing me.
It's the sender's problem - stay out of it. Don't do anything.
Your colleague is going to find soon enough from a number of people that he/she made a mistake. It's a mess and you'd best help your coworker by not adding to their predicament.
The sender is going to have apologize to that someone on the thread that they were snarky to. And pretend that the rest of the world wasn't cc'ed.
I had a similar situation recently, on a smaller basis.
A co-worker did a reply all without realizing that a client contact had been added to the message, and made a somewhat tactless comment about that person. When I brought them some paperwork a few minutes later, I asked if they realized what they had done.
However, if the sender is not someone you work with closely, or there are more than a dozen or so people that the message was sent to, as the other answers have said, I would just ignore it.