The size, channel info etc on the status bar at the bottom is inconsistently updated for the current image. Occassionally it's blank and selecting an image does not update it. For instance, i opened three images by dragging them onto the PI window and the status bar was blank. Selecting images did not update it. I then dragged two more images onto the PI window and the status bar updated. However, it was not updated when selecting different images - just stuck with info for one of the images.
The undo button does not always indicate the processing step for the current active image. I sometimes have to select first another image and then back to the image i wish to perform the undo. Then the undo icon will show the last processing step for the current image. The ctrl-Z command i think always performs the correct undo, so it looks like just a display problem.
Last really minor bug is a window management problem. Occasionally when the PI window is in the background, selecting it will not cause it to be the active foreground window. I first have to click on another (non-pi) window and then select the PI window. (GIMP does this too, so i presume its a windows thing that has to be carefully managed.)