Adding comment fails with incorrect error message when the text content is too large

• Copy the text in the attached file, and paste into a comment on an issue.
• Click the 'Add' button

The action fails, and gives the error "The issue no longer exists."

Two problems:

1. I have lost my text in my comment - if there is a limit on the length of a comment, then it should be clear in the UI
2. The error is wrong - the issue clearly does still exist.

URL = https://qa-jac.atlassian.com/secure/AddComment.jspa?atl_token=AKVY-YUFR-9LM7-97AB%7C9219f1c1e02fe896bdd0c5057ba1252110ca390c%7Clin
Browser = Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:8.0.1) Gecko/20100101 Firefox/8.0.1
Version = (v5.0-rc2#710-r167904)

Workaround for 5.1.0:

For comments, be sure to click the Comment button at the top or bottom of the page. Do NOT use the . quick action gadget
For transitions or other actions, right-click the transition name and select Open in New Tab from the context menu.