With the leftover dinner, still slightly warm on the table, placing it simply in the fridge, maybe tempting as you are cleaning up. But there is a reason why refrigerator manuals warn against it. Placing hot food in the refrigerator may raise the temperature of the fridge as well as other already cooled food. This gives an opportunity for microbial growth as the compartment is not cold enough.
Environmentally, with the warm leftover, more energy is wasted as the fridge tries to lower the raised temperature to one that was originally set.