Editing a post sometimes makes some whitespaces and line breaks to disappear from it. I haven't tried to localize it, so I don't know the pattern/precise steps to reproduce the issue, but it's rather stable.
Sometimes editor puts excessive line breaks to the post. Again, I haven't tried to localize it, but it occurs pretty often.
Copying & pasting a quote completely screws up the way it displays. Stable.
Please, add a line break after a pasted quote, outside of the quote's area, so that users could type "under" the pasted quote. I know it feels like a minor issue from a dev's perspective, but it's really annoying that you keep typing inside a quote if you paste it to the end of a post.