mspaint.exe.... let's please not set the bar so loooooooooow....
Macromedia fireworks, photoshop, and ifranview all save png files with the "phys" block in it.
mspaint.exe and paint.net do not save this block.
Unfortunately, some apps (esp for web rendering) look for it... and some blow up when it isn't there.
Some example png files attached (see the file names for explanation). You can open the files with a hex editor and look for the "phys" tag (only present in the fireworks and ifranview examples -- oops, can't add a 4th example, so no ifranview example on this post).