I think that all the employment I've ever had has had the expectation that I be adaptable. In formal contracts, it may be expressed as "and other duties as may be required", but whether or not it's in a formal contract, I have always felt that I need to be prepared to do whatever needs doing as part of my job.
I have always felt that that expectation was especially valid for teaching. However, now that I'm not teaching, I've discovered that this expectation is in other places as well. I currently do three days a week as a bookkeeper and two days a week as a legal assistant. However this week I got to where I do bookkeeping to discover that the manager is on holidays this week, which means that I really can't do any bookkeeping because he has to sign off on all invoices before I can enter them into the system. However, the cook was sick on Friday and was sick again Monday and today. So, without any warning, I became chief cook. (Rose is chief dish washer.) I made up the week's menu, went and bought groceries, and Rose and I cleaned out the two walk-in fridges of all the disgusting, who knows how long they've been there, leftovers. The regular cook is supposed to be back on Thursday (I go to my other job Thursdays), but I've got easy meals planned for Thursday and Friday anyway.