I spent too much time in my introductory economics classes trying to cover too many topics. Here is a good example.
I covered this topic in all my classes. I wish I had gone through it more, repeatedly, and made sure every student understood it well enough that they could explain it to their parents. From Scott Sumner:
One of the basic principles of public finance is that it makes no difference whether a tax is legally borne by the buyer or seller. The burden of the tax will depend on the relative elasticity of supply and demand, and the economic incidence of a tax doesn't depend at all on the legal incidence. Stephen Gordon did a post discussing this issue, and provided a nice set of graphs for comparison:
In this case most of the burden of the tax falls on buyers, in the form of higher prices. But his post also shows the opposite case, which is more likely to apply to labor markets (where sellers of labor, i.e. workers, are believed to absorb most of the burden of a payroll tax, whatever its legal incidence (which is 50-50 in the US.))
I have often summarized this result as the "Bill Murray" result, quoting from his chant in the not-so-famous camp movie, "Meatballs",
It just doesn't matter! It just doesn't matter!
If all students learned this, and if they could see that it doesn't take long for the new equilibrium to be attained regardless of where a tax is imposed, they would likely understand a heckuva lot more economics.