Posted this on the Adobe forums; thought I'd add it in here too.
Hi There: I don't remember if this was a problem in LR 2.x or not, but it was a bug in LR 3.x and it's still a problem in LR 4.1. The problem is with the filter bar.
Let's say that I have a number of images to process, and I've marked them all with a blue label. I use the attribute filter to show only blue-labeled images. I select the first one, press D to develop it, and after processing I assign it a purple label. Because it no longer has a blue label, it no longer fits the filter criteria and it drops off the screen, bringing up the next blue-labeled image for processing. That is as it should be.
However, if while the next image is loading my mouse cursor hovers over the colour labels in the bottom toolbar, Lightroom will assign a colour to that image, drop it from the filter and begin loading the next one. It will keep doing this in sequence as long as my mouse cursor is there, and the colour labels vary - i.e. one would expect the (arbitrarily-assigned) label to be purple, but it might be red or yellow or...
The same problem exists with with star ratings. If I set a filter of stars = 0 in the filter bar at the top (not >/=) and use that attribute to filter unprocessed images, when finished processing if I give an image say three stars, it will drop out but if my mouse hovers over the star ratings it LR will continue to assign one, two, three stars to the next image, and so on. When I move my mouse outside of the attributes on the toolbar, it stops doing this and loads the next image that fits the filter criteria. It doesn't do this all the time, but frequently enough to be frustrating as I must then hunt down the images that have been randomly filtered and restore them to their original values.
Two things to note: one is that this happens in the Develop module, and the other is that it's not necessary to click to assign a value (beyond the first one), simply hovering over the attributes is enough.
As I say, this has been happening since at least LR 3. Windows 7 Home Premium, LR 4.1, 32-bit.