Well, the best I can do is give an example of what I mean.
One unrelated example was working at Six Flags: no matter how many rides I was trained to operate, I still got paid $6.50/hr...even though I was older than my peers and my bosses. That is taking on unnecessary responsibility without compensation.
The other example (being taken advantage of, and being denied compensation): I worked at a university's IT Department doing User Support, and the main task was fixing computers for professors, faculty and staff and setting up computer labs. That was all well and good, but when I got brought over to the Law School to work in their IT Department at random times, I started getting tasked with solving server and network problems when we'd normally ask the guy labeled "Server Administrator" or "Network Administrator" to solve them. I didn't mind doing it because I wanted to learn how and wanted to showcase my abilities to fix things beyond user's problems. I asked for a promotion to become a server admin and was declined. Not only that, when raises went out (we were paid hourly), my raise was less than half (like $1/hr for them, $0.45/hr for me), when everyone started literally the same day as me (or more recently), and I was the one the others were asking for help to solve their tickets. That is when I realized I was being used for my abilities and underpaid. So I started looking for a new job in the area I wanted to move into since I couldn't get promoted into it.
This isn't to say it'll happen to you - but it's something I am always weary of now, so that I don't set a precedent (in my line of work) that I am willing to give anything to a company without being compensated for it - even though I love what I do. I hope that doesn't sound too negative.