Some people can't execute a procedure

USER_A cannot execute a procedure owned by USER_B when one person logs in.
USER_A *can* execute a procedure owned by USER_B when a different person
logs in.

One person is in one city and the other is in a different city, but in TOAD,
both users can "see" the procedure when browsing the procedures of USER_B.

Furthermore, this is only the case in one environment (DEV) and works
normally in other environments (i.e. QA)

I have confirmed (in TOAD and SQL*Plus) that USER_A has EXECUTE privs on the
procedure granted by USER_B.

Help.  What else can I try?

Other related posts: