Stop the presses is an idiomatic exclamation when significant information is discovered and specifically intended to stop publishing the latest edition of a newspaper because there is a late-breaking news development.
When content needed to be revised just before, or during its printing, the press was stopped and content amended, by changing the plates, type or font, before restarting it.
Reasons to stop the press might be to add a news item or correct an error. This could cause delay and expense, such as by necessitating that already printed copies be discarded, so was generally reserved for highly significant content changes such as the breaking of a very important news story.
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