Alex Chong

I've seen it with a grass block that just got picked up (haven't seen other blocks being carried).

Can confirm. Also win7, java7u51 64 bit.

Ah... I see. I was thinking in terms [r=] being centered at the command block also applying to other selector terms. Nevermind then, that would be a feature request and not a bug.

replaces prison cell with fireball cannon

The given solution only works if the command block does not move. Minecart command blocks and those from /clone will still affect the original target.

I came across this problem while building a UFO that flies using /clone to move itself, /tp to move entities and /fill to remove the old copy. I want to add a tractor beam to suck up cows below (/tp) but there is no easy way to select a 3x20x3 pillar BELOW the ship.

@mohd I thought so too, but I checked the source code and the zombie is not cured, it is killed and replaced with a new entity. The information transferred is only the position and rotation. Also, this occurs with natural villages too, not just cured ones.

Also, do bug reports get rechecked periodically after being closed?

Happened to me four or five times in a survival world, 1.6.2. Kept spawning new ones, but they keep on vanishing. Gave up after a while.
Frankly, I'm not surprised it can't be reproduced. It seems very random and spontaneous. I went for about a week before it started up, and even then it was sporadic.
Edit: not absolutely sure it is caused by despawning; however, I have never observed the disappearance so it might very well be.
Edit edit: I agree with the poster below. This is a long-standing issue which can ruin the fun of a well-built village. It has been reported over several versions by numerous players, and it needs a bit more attention. (I'm not saying it should be easy to fix or anything, (I checked the MCP decompiled code and can't figure out what it could possibly be) just that it is still an issue.)