The test added in bug 1747105 highlights that the source text content for iframe is possibly broken on reload.

Instead of seeing the html content we see Error: Incorrect contents fetched, please reload., which is probably a dead end as we just reloaded.

Note that it doesn't seem to reproduce with a simple data URL equivalent:
data:text/html,<iframe src="data:text/html,foo<script>console.log('bar')</script>"></iframe>
So there might be something special in the test itself.