I did it once when my company was transferring our contract to another company and they needed to test whether the new servers could run under stress

I don't know whether it counted as volunteering since I was salaried, but yes it was boring as hell.

However, there was a benefit I didn't plan on; the non-technical rep the other company sent over to watch from our end took us out for a steak dinner afterwards, and then twisted our arms to go to the topless bar on his dime

oh the humanity