Just had a terrifying moment when, after attempting to install BusyBox on an Android device, almost all console commands seg faulted. This is possibly because I ran out of space on the /system partition, though I'm not certain. (To fix that simply requires deleting some things you don't want, like .apk files for apps the manufacturer/carrier dumped in /system/apps.)

To fix it, I found out that the /system/xbin/busybox executable which was installed was the wrong version/corrupt, so I went to the BusyBox site to download a (more?) correct version; I used the armv6l app for a Samsung Galaxy S2. Then one needs to replace the existing version with a working version. (The following is based on this.)

On your computer, go to the directory containing the file 'busybox' which you just downloaded, and run

adb push busybox /data/local/tmp/
adb shell

Now there is a console open on screen which is actually on the phone. Type

su
chmod 755 /data/local/tmp/busybox
/data/local/tmp/busybox mount -o rw,remount /system
/data/local/tmp/busybox cp /data/local/tmp/busybox /system/xbin/

If one hasn't already installed BusyBox, you can also issue

/system/xbin/busybox –install -s [INSTALLATION DIRECTORY, PERHAPS /system/xbin/]

which sets everything up - BusyBox installers work by creating a folder on the device filled with executables (or better, links to executables) which run /system/xbin/busybox itself. (For me, this was actually at /sbin.)