When 2 users try working on same issue at same time one of the users who is trying to modify a stale version of issue will get a very ugly error screen with no clear indication of the cause of the error.

use case:
(assume both users have permission to modify X-1)

0. issue X-1 is in Open state
1. user1 opens issue X-1
2. user2 opens issue X-1
3. user1 updates X-1 by clicking on Start Progress action
4. user2 is not aware of user1 clicking Start Progress, user2 has not reloaded the web page view of X-1, so the issue still appears as Open. user2 tries to start progress also.
At this point user2 gets ugly error screen.
Instead user2 should get a polite message informing him/her that the issue has been modified by another person and that it should be reloaded in the browser before proceeding with changes.