Apple has touted its group messaging feature as a handy way to communicate with multiple people at once, but blogger Cory Watilo points out one of the flaws in its system: not everyone realises they're replying to everyone in the thread.
The problem comes up if you don't have group messaging available, or if someone in the group doesn't have an iPhone. Here's Cory's take:
If you don't enable Group Messaging, messages from anyone who replies will be sent as text messages to everyone on the thread. But what's worse: your reply will, unbeknownst to you, be sent to everyone on the group message. The problem is that there is absolutely no indication your reply will be sent to anyone other than the person you're replying to. If you don't have Group Messaging enabled, it's pretty cut and dried: your reply should not be sent to the entire group.
It's certainly a handy feature when you're talking with multiple people and they know what's going on, but we've seen our fair share of accidental reply all texts since iOS 5 was released. If everyone is on an iPhone with iOS 5 installed it's pretty clear what's happening, but if you're not, you might not be aware your message is going out to everyone on the list. For now, the lesson is to be careful when replying to obvious mass text messages, because as he notes, disabling group messaging doesn't appear to disable the reply all function.
The biggest iOS 5 bug you've never heard of [Cory Watilo]