That bug has been described on https://launchpad.net/ubuntu/+source/gedit/+bug/82539

"Binary package hint: gedit

Ubuntu 6.10, x86, gedit 2.16.1

gedit doesn't close binary files properly. I tried to open (drag and drop) files from mounted CDROM in gedit. gedit warned me that it cannot detect character encoding, it offered some character sets. I selected CANCEL. Latter I found that file is still open in gedit when I was unable to umount CDROM. I used gedit to check files on CDROM, I was looking for some text files to read, I was not interested in binary files.

$ lsof /dev/cdrw
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
gedit 29196 oem 16r REG 22,64 10648 1871 /media/cdrom1/isolinux.bin
gedit 29196 oem 17r REG 22,64 2048 1863 /media/cdrom1/boot.cat
gedit 29196 oem 18r REG 22,64 10648 1871 /media/cdrom1/isolinux.bin

$ ps aux | grep 29196 | grep -v grep
oem 29196 0.1 1.5 66404 23696 ? S 20:20 0:04 gedit

Steps to replicate:
1) open some regular text in gedit; this step is needed to have gedit copy up and running
2) put CDROM with some Linux to your CDROM
3) Ubuntu should mount CDROM
4) start new copy of gedit from menu, we will use this new gedit for our demo
5) open CDROM folder and 'drag and drop' some binary file to gedit
6) gedit should display message that it "cannot detect character encoding"
7) select CANCEL
8) I expect that gedit will release binary file after CANCEL but it seems that file is still open. Test with lsof tool."