I agree with Erik's post. I have seen this, not only with the GH2 but other cameras if you try and lighten underexposed areas too aggressively in Lightroom either with the exposure adjustment or the shadows adjustment (and particularly if both are used).
If, on the other hand, those spots are on the original unadjusted raw file then there may be a sensor problem, but my best guess is that the original images were underexposed and brightened to breaking point.