Instagram is currently testing a feature that identifies users who take screenshots of public Instagram Stories. Now, we're all partial to stalking, screenshotting content and posting it in WhatsApp groups to laugh at...however, next time you take a screenshot or screen recording, the person who posted the story will be able to see it.
There was significant uproar on Instagram when some users detected the new feature. People even indicated they had switched from Snapchat to Instagram because they wanted to get away from screenshotting alerts. Instagram is likely testing how people respond to the new notices and may decide to never rollout the feature to all users, or perhaps change its behaviour.
In the meantime, be careful and watch those trigger happy fingers.