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.
The default excerpts in WordPress are, in my opinion, a bit crap. All images and subheadings are stripped out and it always stops in the middle of a sentence. It also ends in that “[…]” which I don’t like.