Comment by Stratoscope

Even though the Oxford comma has fallen out of fashion, there are a few cases where you really need it. My favorite example is the old joke:

There are two hard problems in computer science: naming things, cache invalidation, and off by one errors.

Leave out the comma and it makes a mess of things:

There are two hard problems in computer science: naming things, cache invalidation and off by one errors.

Don't take my word for it. Read both versions out loud, pausing for each comma, and not pausing where the comma was removed.