iFrame failing to display when content is published

I've had this situation a few times now with Drupal where inserting an iFrame when editing/creating content, resulted in the src tag being stripped out of the iframe when previewing or viewing the node. The strangest thing is that resaving the content multiple times, or refreshing the page multiple times, seems to make the iframe suddenly appear.... After which point it will appear every time you subsequently load the page.

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