There is a pretty serious bug in the last couple of releases. It appears that the changes I made to support Multi-Site affected the Jobs module.
It appears that jobs that have already been committed to one user appear as available to all other users.
I am at a Lacrosse tournament and have limited access from my phone to be sure. For now, I recommend putting your Jobs module in “Admin” sign up mode until I investigate later tonight. Use the Jobs tab on the Options menu to disable User job sign up.
Update 5/13/2012: This bug has been fixed in v1.27, please update ASAP.