I consider myself a pretty meticulous cook as far as cleanliness and safety are concerned. I'm particularly obsessive about my grinder due to horror stories in the news every so often. Or at least I thought I was. Last night I ground up some beef chuck for hamburgers using my KA grinder attachment. Afterwards I meticulously cleaned it, then ran it through the dishwasher, then sanitized it again and let it air dry. I know you're not supposed to put it in the dishwasher, but I have for a long time and I've seen no ill effects. I gave the mixer a cursory wipe down as well.
Fast forward to this evening. My wife had promised she'd bake a cake for a Church luncheon tomorrow. So I measure out all the ingredients and put all the wet ingredients into the bowl of the mixer. (Wait, didn't you say your WIFE promised to bake the cake?.... Yeah, we all know how that goes) Anyway, I lock the bowl into the base, put on the paddle attachment and clunk the mixer head down into place. Luckily I was looking, as a pea sized chunk of something fell right into the contents of the bowl and immediately sank. I strained the liquid and found what looked like a chunk of beef staring up at me from the strainer. As far as I can tell, it was flung astray by the grinder and stuck to the underside of the mixer head. The wet ingredients immediately went down the drain, and I spent the next hour and a half rendering an already clean kitchen operating room sterile. I was more than a little freaked out at the prospect of feeding members of the congregation cake with chunks o'beef as an unintended ingredient.
Moral of the story? Even things that don't come into direct contact with food can still pose a threat of physical contaminants falling into your food. Range hoods, the underside of cabinets and yes, mixer heads, need more than a quick wipe down.