As with other vehicles, especially Ford products, lug nuts were made of a combination of two metals, the steel lug nut and an aluminum alloy covering. Over time, the aluminum covering swells, apparently due to the incompatibility of the two metals, water getting inside the cover, etc. When the lug nut coverings swell, it is difficult to remove the lug nuts because the standard socket (in this case a 19mm) will not fit.

I found this out when I went to have my tires rotated and was told the technician couldn't proceed without damaging the lug nuts. They wanted to replace all of the lug nuts at a cost of $100+ labor and $1 each for 20 lug nuts. I left the facility and called the Lincoln dealer where I bought the car and in the end Lincoln Motor Co. refused to remedy the problem. I ordered lug nuts from ebay, hammered the socket on each of the lug nuts in order to remove them, then replaced them (one at a time) with the new lug nuts.

I was about to leave on a long-distance trip and feared having a flat tire that I wouldn't be able to take care of unless I changed out the lug nuts. In the end it was a minor out-of-pocket cost to me. I imagine the problem is so wide-spread that it would cost the automobile manufacture a lot of money to replace millions of lug nuts, so they just leave the burden of doing so on the consumer.