On Windows XP, I noticed an oddity about Pixelize Filter, which occurs when it is applied with very high values of "Pixel Width" and "Pixel Height" (these two parameters must be approximately greater than half of the corresponding layer sizes).
More precisely, there are never problems when the filter acts upon layer masks, channels and layers lacking in alpha channel.
On the contrary, if the active layer is provided with an alpha channel and you choose large values, as mentioned, of "Pixel Width" and "Pixel Height", then some of those color blocks produced by Pixelize seem to be eventually much darker than normal, as well as endowed with mistaken hues. At the most, after setting the values of "Pixel Width" and "Pixel Height" equal to the corresponding sizes of the layer, we will get a single color block that appears too dark (often almost black) compared to what should be.

For instance, the situation I've just reported can be simply demonstrated, for GIMP 2.6.7 running on Windows XP, by following three easy steps.

1) Create a new image with these options: Width: 640 px ; Height: 400 px ; Fill with: White .

2) Select " Layer > Transparency > Add Alpha Channel " .

3) Apply " Filters > Blur > Pixelize... " with: Pixel Width: 400 px ; Pixel Height: 300 px .

At last, all the four color blocks should be obviously white, but strangely it isn't so, since two of them are dark gray. Instead, if we skip the step #2 so that keeping the layer flattened, we would get the right result.