There has been also discrepancies between image editors.
A PNG in Photoshop looked different than the same image in PSP. Something to do with PSP never showing the alpha and PS showing it always. PNGs created by PS were bloated so I always had to resave them under PSP, that's where I noticed it.
Amusing (or maybe not so amusing) fact: Painter never supported the PNG format.
And PSP always stripped out the alpha when saving, I believe. That washed-out look was always gone after a resave.
Three image editors, three broken PNG support.
I loved PNG for its compact size but I never got to use its alpha.