I'm the gamemaster676 he was talking about. I have used a skeleton spawner to make a xp grinder, and I made the walls out of obsidian. To prevent monsters from spawning while I was working I placed a lot of torches on the walls, but everytime I removed them to test, one (and the same) obsidian block kept disappearing while I hit the torch with my fist.
The block didn't break but was just gone.
This was happening on a SMP server without plugins or mods on my side.
But that does not really make sense. In this (http://youtu.be/bYwzd62cLSw) video by Etho, he shows a hopper timer build in one of the first 1.5 Snapshots. If you build it now in the 1.5 Release, it doesn't work anymore because of this bug.