Urban dictionary suggests an appropriate term: demo effect.
Its downside term is that is typically connected with high-tech, and it has other meanings.
From another source:
Similar to [the bonaldi effect], the demo effect: an object, software, etc., used daily with no incident, would present a dysfunction during a demonstration, especially in public. exemple [sic] : bill gates' presentations of different windows, ending with a famous error on a blue screen.
