You spend 20 minutes on creating a detailed issue / long comment, you somehow navigate away and everything is gone. The user motivated to give up never filing the information or only file a one-sentance issue.
The 'Browse to attach images or drag them into the comment field' did not capture my file and the browser instead opened the file (browser issue, need to hold the file for 1s), you could accidentally close the tab, you could press back/forwards with mouse buttons, browser might close due to anything.

Save the comment (as a draft) separate of the pages. For example:
I have open:
- pagure.io/pagure/new_issue
- pagure.io/flock/issue/226
and say I had fedoramagazine.org open, wanted to close it with Ctrl+W, but instead pressed Ctrl+Q, quitting the whole browser. Normally it wouldn't be a problem, but after re-opening the browser all unsubmitted issues and comments are gone.