I mistakenly bought dog food on sale that I thought was cat food. It didn't say dog on the label and they are small cans like the ones I buy for my cats.

All right, one can won't hurt but your cat may not like it. If you have the sales slip, or used a shopping card, often you can return it for a swap.

The issue with dog food is that dogs can make taurine from their diet, and cats cannot. Therefore, dog food has no taurine, and cat food does. But lack of taurine is not an instant death for a cat so a small amount once or twice is not going to be fatal.