A while back, there was a "Vexer 33" in which I asked a rather interesting
question on why, two different methods of determining the energy of an
electrostatic case for an image charge system gave two different answers. You
can find the question in the Files section of the Undernetphysics group webpage.
The answer was posted somewhere in the old archive messages.

In any case, I found it interesting that the same issue was tackled in a recent
publication in Eur. J. of Phys.

M.M. Taddei et al. Eur. J. Phys. 30, 965 (2009).

I think you can get free access to it within the first month after it appeared
online.

http://www.iop.org/EJ/abstract/0143-0807/30/5/005

Of course, this pedagogical paper also deals with more general situation. Still,
it's fun to see the same issue that we dealt with in one of our vexer appearing
again.

Zz.