IPhone – Why does Safari on the iPhone sometimes display a broken play image for YouTube videos

Here's an estimation:

The site you're likely viewing has what is called a liquid-layout, which means that the content will spread to fill it's container. So when you're rotating the phone, the viewport width is changing from the narrower portrait width to the much longer landscape width. So the text itself might not be changing size, the webpage is just adapting to fit it's new window size.

Luckily the above link also has a liquid-layout, so you can see this in action. Take the window on your computer and shrink it's width. You'll notice that the text will shift around to fit it's new container size. So it's the web page (more specifically the CSS) which determins how the text wraps.

tl:dr

CSS did it.

I would imagine this has something to do with Core Data, memory warnings, or both. If Apple is using its own kit, then it is saving the cache in a Core Data store, which is, essentially, an unordered set. My theory is that when you go to a site, Safari is going to the Core Data store and grabbing the first instance of the URL in question if it exists, which can be old, new, whatever. But since I also don't think that Apple would be nearly that sloppy, I'd say that every page attempts to cache, but the amount of data required to properly cache a site is beholden to memory warnings, which you, as the user, should never, ever see, only this time the newer page simply doesn't get cached and that's why you get random older pages in view.

Okay, that was a bit technical. In short, I think Safari attempts to maintain a clean cache, but memory limitations of the device don't always allow that to happen.