This problem is in an up-to-date 12.04, and seems to have started recently.

I have my .bash_history file set to read only. Lately I've noticed that despite this the file is being modified !
Here's the scenario.
In a new terminal, here is the tail of the file:

$ tail -5 .bash_history 

mkdir -p ttt ; gs -dNOPAUSE -sDEVICE=png16m -r180 -sOutputFile=ttt/p%03d.png *.pdf

rsync -av /home/u1204 /media/SEAGATE/u1204 

gnote.late.reminders|GREP_COLOR='1;32' egrep -i "is overdue"

pvr_iplayer -x -HD /home/u1204/Documents/pvr/tv-hd ; pvr_iplayer -x -STD /home/u1204/Documents/pvr/tv-std

exit

open a shell with raised privileges

 $ sudo bash [sudo] 

 password for u1204: 



 # ls -l ~/.bash_history

 -r-------- 1 u1204 u1204 1266 Feb 23 19:52 /home/u1204/.bash_history

so .bash_history is still definitely read-only.
Do something, then exit the escalated shell:

# echo "written to history file"

written to history file



# exit

exit

And Voilà, the history file has been modified !

$ ls -l ~/.bash_history

-r-------- 1 u1204 u1204 1324 Feb 23 20:10 /home/u1204/.bash_history

Open another terminal to confirm:

$ tail -5 .bash_history 

pvr_iplayer -x -HD /home/u1204/Documents/pvr/tv-hd ; pvr_iplayer -x -STD /home/u1204/Documents/pvr/tv-std

exit

ls -l ~/.bash_history

echo "written to history file"

exit

I'm pretty sure that it didn't used to do this (in 10.04 at least).

Anyway, this can't be correct behaviour, can it?