Why would a car smell like something burning after sitting at a light with the heat on?

A car may smell like something burning after sitting at a light with the heat on due to several reasons. One possibility is that the heater core, which circulates hot coolant, could be leaking, causing coolant to burn on the engine or exhaust components. Another reason might be debris, such as leaves or trash, that has accumulated around the engine and is overheating. Additionally, worn brake components can generate a burning smell if they are engaged for an extended period while idling.