Haircut

The amazing haircut machine made barbers obsolete.
All you had to do was stick your head in a box, and the computer-scanners would figure out the perfect haircut for you.
Five seconds with a series of lasers, and you were done.
Okay, so there were a few glitches in the system’s development, but those prisoners were too dangerous to have their hair cut by any other means.
No matter how well you chain them up or incapacitate them, putting a prisoner in close proximity with someone wielding a sharp object is a very bad idea.
A little off the top?