When visiting the last post of a thread, dynamically-sized embeds (such as tweets) from previous posts will expand upward even if they fully appear on the screen. It will keep the post the user is reading at the same position on the screen and prevent the useful content from moving around as the result of other elements loading.
A bug has been fixed that caused the lazy loader to incorrectly determine which way the page was being scrolled. It didn't really have any impact on the user experience and is only mentioned for completeness.
At this point I'm happy with the behaviour of the lazy loader and I don't have any more changes planned. The lazy loader is meant to be almost invisible so I may write a full description of the multiple ways it improves the user experience behind the scenes.