Reproduced in Chrome / Firefox on Linux, JIRA 5.2.1
Pre-requirements: JIRA 5.2.1 (or 5.2.X) with installed GH and Attach Image for JIRA plugin.
1. Open Green Hopper board for a project.
2. Click on some issue from the backlog.
3. Click 'a' key on keyboard to trigger the Assign dialog.
Result: nothing happens.
Expected results: assign dialog is shown.
NOTE: the expected behavior is not reproduced if the "Attach Image for JIRA" is disabled.