When making a new post one day I came across a bug in my website. I created a new post, attached a featured image, and hit preview. I got a messed up page that had two copies of my featured image above the page content.
As panic set in I went through and checked everything. My site still looked fine. Already published posts seemed okay. Checking previews of already scheduled posts showed that they were fine. I tested different images to see if they were the cause. No luck there either.