What really grinds your gears?

When people choose not to comprehend something. What I mean is people who have something explained to them (verbally, written, however) and they actively choose not to comprehend it or just choose to ignore it. Example would be where I work in an ER and we have certain policies that people are expected to follow. We explain them over and over and have some of them displayed in large signs and every day multiple times we have to argue with people over something we already explained.