At work we have a very simple microwave with only 4 buttons (not counting the door open lever/button):
This microwave manages to be one of the most effective user interfaces I've ever come across. Pressing the plus ("+") button ups the time in 10 second increments, until you reach 90 seconds, and then it ups the time in 1 minute intervals. Once you pass 90 seconds, the display shows whole minutes. You can't set a time of 17 seconds, for example, or even 6 minutes and 30 seconds.
Pressing the minus button decreases the time by 1 minute if the current time...