I have gotten into the habit of scanning the whole email thread before I forward it to more recipients. There were few times when an email started with very small audience and they were quite open in discussing the company’s strategy and roadmap (staff cut and consolidation), then over time more and more people were added to the thread, before you knew it, what was meant for a select few just became public knowledge.
Of course, by scanning a long thread also gave me a lot of inside information and source of amusement.