I have not shot any examples with both a 12bit and a 14bit camera, but some example pictures where I hit the quantization limit of even the 14bit DNG from the K-5 come to mind. A recently posted one in Project52 was this picture after significant post-processing:

Processing with a flat profile (attached) clearly demonstrates that there is extremely limited dynamic range (significantly less than 2EV) to work with here - basically a thick layer of spray. In addition, I had to underexpose because I could not change lenses (spray and sand blown), the DA 18-135 was already at its widest, I did not want too much blur in the waves and anticipating having to steepen things up wanted to use a fairly low ISO to avoid amplified noise later. This way, I gave up nearly ~2EV at the top end (2bit) by not exposing to the extreme right. That leaves 12bits to start with. Then, considering the main information in the image is spread from around 80% to 130% of the average only, there's another bit gone. Now apply a nonlinear curve to the remaining 11bit of range and you start to see quantization artefacts in the resulting 8bit JPEG image. E..g., I could not get any better contrast in the clouds without posterization.
This shot, taken with 12bit raw data could only be presented with an almost flat sky - very different from what the intention and visual perception at the moment in time was. There was so little color differentiation even with 14bit raw data that I refrained to present it in color altogether.