Currently if two users update the same issue at the same time, one set of changes is silently lost. Changes are lost even if users update different fields, as long as user2 starts his update before user1 submits his.

I propose refusing update if any of the fields being updated have changed in the DB since the original values were retrieved. This involves 1) figuring out exactly which fields were updated by the user, and 2) figuring out whether they changed outside of this session. Both can be done by saving the original values (or their checksums), and comparing to the values in the database right before the update, preferably in a single DB transaction.