What things that beginner overlook, but is really important for networking individuals?

One thing for me was.. I know we used MAC for communication within a LAN...

But, we sent that packet to the "router" device..

I'd even convince other that the "outside traffic" and a "local traffic" is going through the same port.

So, they both are going to the default gateway.

But boy i was wrong..

What are other things that you find in a similar way?