Interstitials Blocked with Robots.txt Might be Seen as Cloaking
Mobile Interstitial Penalty is Calculated on Recrawl
The mobile interstitial penalty is calculated in real time as pages are crawled.
Mobile Interstitial Detection Will Improve
Google will be improving the interstial classifiers over time as they find more examples, so any interstitials which are not affected yet might become an issue in the future.
Mobile Interstitial Penalty Will Roll Out In Days
The Google mobile interstial penalty will roll out to all data centers in the next few days.
Use 503 Status if Serving Pages with a Temporary Interstitial
If you are returning a conditional or temporary interstitial or modal window, return a 503 status instead of a 200, and Google will ignore it rather than index the content of the interstitial, and you won’t get an interstitial penalty.
Interstitial Penalty Only Affects Landing Pages
The interstitial penalty will only apply to landing pages. Any actions which occur after the landing page visit are OK.
Interstitial Penalty Only Affects Mobile Pages
The interstitial penalty will apply just to mobile pages at the moment.
Event Based Interstitials are Not OK
Use Fetch and Render to Test Overlays
If you have some kind of overlay or pop-up on landing pages, you can check what it looks like to Googlebot using the Fetch and Render tool in Search Console, and check the impact on engagement using web analytics.