Stopping down and diffraction?

Do you get noticeable diffraction at larger apertures? f/16... f/22?

Or if it "safe" to use them?

I've never noticed problems at f/16 or f/22, sometimes you just need those apertures to get enough depth-of-field (in macro photography, for example).

At very small apertures, your photos will become less sharp because of diffraction.

Does anybody have an example that shows this?