The ones I saw, they do not only do that when a demo fails. Almost every single part of a demo runs on a different machine. That minimizes the risk that your demo, by a series of minute deviations from the script, ends up in uncharted waters (this is especially true in alpha or beta software). It introduces a risk of inconsistencies between scene cuts. Those you can minimize by making sure that the 'scenes' at cut points aren't too complex, and by rehearsing, rehearsing, rehearsing.