I often read that PNG is a lossless format so I decided to do a little experiment to try to prove it. In Photoshop I did Save for Web to PNG-24. Then, I did File->Place to open the PNG and import it to the original file as a new layer. Then, I changed the blending mode to Difference. If the two layers were identical the appearance should be all black. It wasn't. I tried various options such as converting/not converting to sRGB, including transparency or not. But they all show a difference.
I would expect a truly lossless compression algorithm to yield an exact copy of the original. Is this a problem in the PNG format? A problem with Photoshop's implementation of the format? Or what?