If a user is typing a Forum thread and they decide to click the 'View Full Description' link for the topic, the page refreshes and they lose everything they typed.
I duplicated it on our 2.9.1 instance and also here http://nightly2.sakaiproject.org:8082/portal/
Steps to duplicate:
1. Create a Forum Topic with a description.
2. Click the Topic.
3. Click 'Start new conversation'
4. Enter some text in the the textbox.
5. Click 'View Full Description' and your entered text is lost.
Toggling the 'Hide Full Description also has the same effect.
We have received complaints from frustrated users who have lost work due to this, for that reason I put the Priority at Critical.