Those K&N air filters that everyone love to put in their cars. They improve air flow, but you guys realize they pretty much also let all of the dust through right? I used to work as a mechanic and like when people don't want to change their air filters, I would just blow them out for them and shake them out, and you see tons and tons of dust comes out. I try to clean out those K&N air filters and literally not a speck of dust comes out of them when I try to blow them out, lol. Wonder where all of that dust is going...
Edit: Just to be clear, obviously everyone can do whatever they want to their cars, and yes I do acknowledge they do increase air-flow, but there's always a trade-off that the manufacturers of these parts don't bother to mention...