What is a "bus error"?

[John W. Baxter]

In article
<Pine.LNX.4.43.0202011202560.4970-100000 at zeon.well.ox.ac.uk>, Kenny
<kccw at well.ox.ac.uk> wrote:

>         However, some of these programs now give a "bus error" message -
> what does this mean?

cf:  "What happens if Guido is hit by a bus?"
    (From early Python history.)

   --John (who only did this because you've gotten real answers already)