We have had this situation many times in the past few years. A SQL Developer user has an uncommitted transaction and has left his/her desk for many hours, blocking some other sessions that need to complete a DML. We could kill the blocking SQL Developer session, but normally we don't do it in case the user has run an important job. Other than repeated education to our users, how about setting up some kind of warning in the SQL Developer window? Is there anything like that, a big bright color sign or icon? If not, could this be a feature to be added to a future version?