I have suspected that something is going on with the sequencing of images on Google Image Labeller. Here is some evidence. Often you get two or three images which MIGHT have the same descriptors. For example in the image below, we have a cartoon of the Star Wars Death Star over Europe with the Emperor and a robot, and the immediate next image is a picture of the Death Star from the film. Afterwards there are two images which could be labelled "trees." Because you can't use the same label for two successive images, that could cause a problem. Of course the ignorance of most users foils Google's careful planning.

By the way, if anything this has become more common after the game was updated on March 13, 2007. FrankD

Of course this could be an accident but here are two images, which appeared in succession, designed to elicit the word "hat":

And here are some more: "CAMERA"

The next two are quite recent, post March 13:
DIAGRAM

HOUSE
You can see that the label "house" didn't work for the image on the left because it had already been applied to the image on the right.

JFK

DOG

DRAWING

JET

PLANES

MAP

CHAIR

LEAVES

This one worked. Man, Man, Man, all 3 in sequence in one game.

And this one also worked, Man, Man:

This didn't work as well, Girl, Girl, Girl:

This game had a couple such sequences

SPACE

WATER