Most annoying everyday technology

 The "Check Engine" light.  

It comes on, you pay to have the computer read what the Check Engine light thinks is wrong, but it doesn't seem wrong to the mechanic.  He has to fix stuff anyway because you can't get the car inspected if the "Check Engine" light is on even though no one inspecting the car can tell if the thing that is wrong is serious or trivial.  The computer clears the code that makes the "Check Engine" light go on, and so you can't tell if the "Check Engine" light will stay off or not.  You drive a few miles and the "Check Engine" light comes on again, but whether for the same reason or one of the other reasons the computer considered or for a new reason it is impossible to tell.  The "Check Engine" light is like the Oracle of Delphi, but more annoying.