Lyric discussion by HoppyChris 

Why would you lie about how much coal you have? Lying about how much coal you have can easily be done through the omission of an oxford comma.

An oxford comma is the comma right before the and in a series.

I have 100 pounds of iron, 50 pounds of steel, and coal. I have 100 pounds of iron, 50 pounds of steel and coal.

In the first example, the amound of coal is not specified, while in the second example there are clearly 50 pounds of coal. By omitting the oxford comma, you can let people think that you have 50 pounds of coal, even if you do not, as the oxford comma is often viewed as optional.

But why would you lie about how much coal you have? why would you lie about something dumb like that?

first line of Romeo and Juliet, an English drama:

Gregory, on my word, we'll not carry coals.

also ties into the boyfriend/girlfriend argument

This is the correct meaning. This is the most commonly used example (steel and coal) to explain how the Oxford comma works.

@HoppyChris but "50 pounds of steel and coal" could also refer to a collective lump of steel and coal that collectively weighs 50 pounds.

Consider this:

I have 100 pounds of iron, 50 pounds of steel and coal, and 25 pounds of copper.

What does the bit in the middle refer to? Even though the writer has used correct grammar (recognises Oxford Comma), ambiguity remains simply because of "and".

An error occured.