I prefer plain-text email, and I've observed what seems to be a bug.  When I've been typing a long plain-text email for a while, sometimes the email will be converted to HTML code. The paragraphs go away, the text gets lumped together in one paragraph, and HTML tags such as <div> appear in the text.

Fortunately, there is a simple fix: switch to rich text, which restores the original paragraph structure, and then switch back to plain text.  But it's annoying.