We tend to think of certain common icons as fixed and standardized, but in fact, if you pay attention, you start to notice that very few of them really are. I mean, how could they be? A logo for a company (the mark could be seen as an icon in a sense) is central to its brand identity. Therefore, they have vector art files for it and usage guidelines, etc. It is exact and must never be tinkered with or they risk diluting the brand. Icons (visual symbols, pictographs, whatever) in the world are quite different. Something that I find really fun about them is the endless variation. Its like they are all slightly distorted copies of an original that has long since been lost.
One of my favorite examples is the wet floor sign. I started amassing a small collection:
Of course, this is just one example. This post could go on and on. There are so many to pay attention to:male/female bathroom, handicapped, elevator door open/close, walk/don’t walk characters (I have small collection of those too), even the arrows on highway signs often differ by states.