In 1943, the U.S. Mint made pennies out of steel rather than copper—because copper was being used to make ammunition casings for the war effort. So the 1943 penny is a real oddball, typically called a "steelie." If you've never seen one, this video shows a few, next to some slightly older wheat pennies. Here's what steelies look like:
Steelies, while practical from a copper-preservation point of view, created a bunch of problems. First, the coins looked a whole lot like dimes, so it was hard to make change with them. Second, the steel coins can be attracted by magnets, while the copper coins cannot—this caused many vending machines to reject them, because magnets were used in those machines to reject steel slugs shaped like coins...and that's basically what these were. The final problem was that in some cases the coins would actually rust, particularly around the edges. As a result, the U.S. Mint quickly reformulated the coins for future years, and eventually gathered up and destroyed steelies, making them relatively rare (though not particularly valuable) today. (Steelies go for about a dollar on eBay.)
I have never run across a steelie in the wild. Have you?