[afterburner]

I see many mistaken "Reply"'s every month, and I think I've only seen one mistaken "Reply to all" in my life.

[modeless]

You've never seen an email storm then? They're most often started by a mistaken "Reply all". http://en.wikipedia.org/wiki/Email_storm

[pcl]

In my experience, they've been caused by misconfigured listservs.

[afterburner]

I've read hilarious stories about them, but I've never been part of one. (Someone replied to all once at my old job when they shouldn't have. Then they said oops. End of boring story.)

[wladimir]

On the other hand, the potential damage of one mistaken "Reply to all" can be enormous, compared to a mistaken "Reply" which just adds some extra work.

[afterburner]

Usually true, but I've seen an errant "Reply" cause misunderstandings too (due to lack of communication rather than inappropriate communication).

You're courting disaster anyways by replying off a mass email chain to one person on it.