Bad History when SuperUser.

[Kevin Wang]

Some shells also exclusively read/write to the history file, and not
keep it in memory.  that really makes things entertaining, because
when running two shells running as the same user, "!!" is not always
what you think it's going to be!

for example:
shell 1: ls /dev
shell 2: ls /tmp
shell 1: !! == ls /tmp

Oops!  but it can be a good thing sometimes. it grows on you after awhile.