A bug has been reported in Discourse where expanding a linked article with a onebox causes HTML characters (> and <) to be displayed. To reproduce the issue:
Create a topic with a > or < symbol in the first few lines.
Create another topic and link the original topic.
Onebox the original topic.
Expand the article by clicking the down arrow.
@wesochuck has provided screenshots demonstrating the issue and has offered to reproduce it on try.discourse.org. A fix for the bug has been committed by @eviltrout and is available on GitHub.