The main issue with having separate accounts and instances for Dev and Prod is that for one-stop shops like the one I work it, a sysadmin can still have multiple windows of SSMS open and can still restore Dev over Prod if (s)he is not paying attention.

That's one of the reasons I color code my instances' query windows in SSMS (green for Dev, yellow for Test, orange for QC, and red for Production). The colors help me remember what environment I'm currently connected to. Also, I stop before I hit any okay buttons and verify everything in the windows to make sure I'm pointing to the correct instance.

In the case of a restore, we have different drive letters for all our instances, so it makes it easy to see at a glance if we're restoring to the correct drive letter. If I'm restoring to D: instead of G: for instance, I know I've hit the wrong instance before I even click OK.