Reasons why computer problems seem to mysteriously vanish as soon as a technician shows up:

• You were spacing out and skipping a step somewhere without realising it, and you can’t reproduce it when you try to demonstrate it because now you’re paying attention to what you’re doing
  
  
• It’s an intermittent electrical connection fault that’s being aggravated by movement/vibrations in your desk; you need to check your cables
  
  
• The act of explaining the problem to someone caused you to figure out what you were doing wrong
  
  
• The real cause of the problem was somewhere upstream of your terminal device – for example, at the network service provider – and it got fixed at the source while you were waiting
  
  
• Your computer is in a location with poor airflow and is overheating; waiting for the technician to arrive gave it a chance to cool off
  
  
• Despite all appearances to the contrary, modern computers actually have very good fault recovery, and most minor problems will sort themselves out on their own if you give it a minute
  
  
• Magic