David Drane found that he lost the ability to Double Click and Delete objects.  Here is what he did:

I have been running some third-party LISP routines to assist me in repetitious utilities and found that double-clicking and the delete key no longer worked.  After sifting through lots and lots of variables, I found that two needed changing:

DBLCLKEDIT needed to be set to "on".

And PICKFIRST needed to be set to "1".

Everything works again now.

Notes from Cadalyst Tip Patrol:  These two system variables in AutoCAD are very important to have set to work the way you want.  The DBLCLKEDIT variable controls the way AutoCAD functions when you double-click.  The ability can be turned on or off.  The PICKFIRST variable controls how selections are made.  With it off, you have to issue a command before you can select anything.  With it on, you can select the objects, then start the command.  An example is the delete key.  In order to use the delete key, you have to select an object before you can delete it. Tested in AutoCAD 2009