If you have a site that supports AMP, you may notice that in your Search Analytics report within the Google Search Console, your data for clicks and impressions may have dropped as much as 90%. Google posted on their data anomalies page that there was an issue with them counting the data for the AMP non-rich results impressions and clicks. If you fall within this bucket, you may see huge declines in clicks and impressions in this report.
There was an error in counting AMP non-rich results impressions and clicks. The issue did not affect AMP appearance in search results, but you might see a drop in your data during this period.
The period affected is between December 14, 2017 through December 18, 2017.
Here is what that report looks like when impacted:
Forum discussion at Google+.