is a bug whose presence is affected by act of observing it."

The specific context was a Smalltalk-80 image on a Unix workstation. We noticed that when we left the Smalltalk image running (with the cursor over one of its windows) unattended for a long time (like an extended lunch) and returned, the system was hung and the screen was filled with a huge stack of walkbacks. Yet, as soon as the mouse moved, the screen cleared, everything came back to life, and all was well.