database version: mysql 5.5.24
ruby version: 1.9.3p0
rails version: 3.2.13
redmine version: 2.3

I have been able to reproduce this multiple times, with these steps:
1) User A creates a ticket with Target version of '4.0'
2) User B changes the ticket to have a Target version of '3.2'
3) User A, with the ticket still open from before, clicks the browser's refresh button; User A now has the ticket open with the most recent data, including being able to see the updated Target version of '3.2'
4) User A clicks Update. Target version then gets automatically populated with the stale '4.0'

At this point, if User A notices that it has changed back to the stale value, he can of course manually alter it. But if he does not, and is intending only to make some different change to the ticket, then when he clicks Submit, he will inadvertently save the stale value for Target version.