And while it would be easy for me to type, "No receptionist will ever be asked to give a pedicure to the board of directors!" (and in fact, it was easy), I can foresee a situation where that would be a reasonable request. If it's a beauty related company and the receptionist is, in fact, licensed to do such things, it wouldn't be crazy to ask her to do this. If it's at a plumbing supply company, that would be bizarre.
There are two things that are important to remember. First, nobody can predict the entire range of tasks for every job. You don't know, when you're writing the job description, that your toilet is going to overflow at 10:00 a.m. and the janitorial staff is a contract service that only works nights. You can't go all day with "debris" on your floor, and there isn't a single person in the company whose job description says, "clean up malfunctioning toilets." Somebody has to do it, and it might be you. (Although, I recommend you schedule important off-site client meetings for times like this.)
Second, your job description is not legally binding unless there is a contract involved. At almost all jobs in the United States, you are an at-will employee. You can quit, they can fire you, and duties and responsibilities can shift. I can eliminate your position and put you in a new one without batting an eyelash -- and I have been involved in such changes in the past.
If asked to something you deem unreasonable, can you subsequently quit and receive unemployment? How about a big, fat "maybe"? The state determines unemployment eligibility. If the company doesn't fight it, they tend to grant unemployment. But if the company fights it, it's probably going to come down to whether this was a "constructive discharge" or not. This is where the company makes it so miserable to work there that any reasonable person would quit. Unless the "additional duties" are extreme, you probably won't win. In your examples, being sent to do spot welding without proper training would fall under this category; being asked to deliver a few packages would not.
It's in a manager's best interest to keep duties related to the core function of the job. It's in an employee's best interest to do what needs to be done. That doesn't have to mean you roll over and take whatever they throw at you. You can speak up, you can point out that this isn't the best use of your time, and you can make terrible tasting coffee so they won't ask you again.
And on a random note, when the toilet overflows at an office with no janitorial staff, the person who caused the problem should fix it. If that person ran and hid or denied culpability, the boss should do it.
Readers: What's the craziest "additional duty" you've ever had to do?
Photo by Pink Sherbet, Flickr CC 2.0