The color painted by bucket/gradient tool (maybe applies to others as well, didn't test), shifts by one (almost always down by one).
So, let's say, I painted a background with HSV 0,0,50 (gray) and then do some work. When I select color picker and click on background, it shows V as 49 !!! It is not only for "show". The color really shifts. The same often happens when I switch two predefined colors in the color window. The trend is to go down, not up. (V values tend to go one down).
If it doesn't happen on first try, you're lucky, try a few more times.
Even more disastrous is that this also happens when merging layers, causing me to loose precise work. When I merge layers, two images with 0,0,50 backgrounds, being in overlay mode, suddenly produces 0,0,49 result after overlay mode merging. I had to bucket "several" times to remedy. But, this is no good since the workspace I have generally have very close colors (0,0,49 on 0,0,50 background, for example).
To reproduce this easily, paint a background with #808080 (0,0,50 hsv), select secondary color, bump V by 1 (51), do some random brush on the picture, then try clicking primary and secondary colors a few times to see their values first, then check color picker on background, rinse&repeat.
The more work you do with colors close to each other, the more shifting tends to happen.