I think there's a bug in Hacker News where it will convert new lines to an html p and place it back in the text box if there is an error submitting the form (like your title is too long). Instead of waiting for a successful submission to convert the new lines. That said, this is purely speculation based on my experiences.
Confirmed bug.
To reproduce:
1) Set title to "Ask HN: Attempting to confirm a bug... aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaa"
Set text to: "here is text
Enter pressed twice, I have not inserted any html tags
If this doesn't error and instead posts it, I apologize."
Click submit.
Observe text in the box "text" has changed to:
"here is text<p>Enter pressed twice, I have not inserted any html tags<p>If this doesn't error and instead posts it, I apologize."
To add blank lines in your text output here in HN, hit return twice (\n\n).