I met with some friends one day, we wanted to play some games in a LAN. Three computers were set up in three different rooms. We had to do some configuration (IP addresses and so on), and we sat in front of one computer, trying to click through the setup windows with the mouse. On any click, we heard an error beep, the kind you get when you're clicking outside a modal window (a dialog box that doesn't let you play with other windows until you've dismissed it). On any click. It was really annoying, because we saw no error, and we were able to navigate through the setup windows and get the PC configured.
So we went on to the second computer. There was a modal window open and some kind of error message. That computer had the same type of cordless mouse as first. We had been moving the mouse pointer and clicking on both PCs simultaneously, and the beeping we heard came from the (really loud) speakers of the second computer.