The term “Heisenbug” is a playful pun on the name of Werner Heisenberg, the well-known physicist who first asserted the observer effect of quantum mechanics. “The observer effect (not to be confused with the uncertainty principle) is the fact that observing a situation or phenomenon necessarily changes it. Observer effects are especially prominent in physics where observation and uncertainty are fundamental aspects of modern quantum mechanics. Observer effects are well-known in fields other than physics, such as sociology, psychology, linguistics and computer science.” - The Observer Effect: IEEE publication, K. Baclawski et al. An example would be checking your tyre pressure. By simply attaching a tyre gauge to the valve, you’re almost guaranteed to lose a very small amount of air. So simply by trying to read the tyre pressure, you’ll have changed the tyre pressure. With this in mind, we can assume that the mere process of trying to reproduce a defect could change the behaviour of the code enough that the defect no longer occurs.