Just posted this in another thread, but i think you'll all like it too:

I once had a dev who was allowing his site elements to be embedded everywhere in the world (intentional) and it was vulnerable to clickjacking (not intentional). I told him to restrict frame origin and then implement a whitelist.

My man comes back a month later with this issue of someone in google sites not being able to embed the element. GOOGLE FUCKING SITES!!!!! I didnt even know that shit existed! So natually i go through all the extremely in depth and nuanced explanations first: we start looking at web traffic logs and find out that its not the google site name thats trying to access the element, but one of google's web crawler-type things. Whatever. Whitelist that url. Nothing.

Another weird thing was the way that google sites referenced the iframe was a copy of it stored in a google subsite???? Something like "googleusercontent.com" instead of the actual site we were referencing. Whatever. Whitelisted it. Nothing.

We even looked at other solutions like opening the whitelist completely for a span of time to test to see if we could get it to work without the whitelist, as the dev was convinced that the whitelist was the issue. It STILL didnt work!

Because of this development i got more frustrated because this wasnt tested beforehand, and finally asked the question: do other web template sites have this issue like squarespace or wix?

Nope. Just google sites.

We concluded its not an issue with the whitelist, but merely an issue with either google sites or the way the webapp is designed, but considering it works on LITERALLY ANYTHING ELSE i am unsure that the latter is the answer.