[happytoexplain]

I mean attempting to demonstrate something broken, only for it to work properly.

[copypasterepeat]

Wouldn't this term still apply? If the goal of your demonstration is to show failure, and then that succeeds, that makes your demonstration a failure.

[gmueckl]

That term definitely applies to any demo that behaves differently than previously observed - subtly implying that the demo setting has something to do with it.

[happytoexplain]

I don't how it's used in German, but based on other comments, it does sound like it can be used either way!

[knoke]

As already said: Vorführeffekt is neutral and could mean both: success and failure. But the opposite of „a performance with an opposite than expected outcome“ is something that comes out exactly as it’s supposed to be without intervention, right? ;)