Googlebot for smartphones identified a significant increase in the number of URLs on that return a 404 (not found) error. If these pages exist on your desktop site, showing an error for mobile users can be a bad user experience. This misconfiguration can also prevent Google from showing the correct page in mobile search results. If these URLs don’t exist, no action is necessary.
See erroneous urls here:
yslbwnqu3hclickhole.kinja.com was used as the staging domain and the renamed at launch to clickhole.com.
There were several migration attempts after which the migrated content was wiped out and a new migration was started over. Wiping the content means those posts were deleted and when the migration ran again a new post id was allocated for the same post. We are guessing Google somehow picked up posts during those migration attempts, if bots were not correctly blocked on the temporary blog.
We have completely deleted the blog host yslbwnqu3hclickhole.kinja.com so that we don’t redirect to a 404 page but return 404 immediately for these posts.