A convincing demonstration can rely on conditions the audience does not realize were arranged.

A demonstration is a performance with a setup. What you see may depend entirely on conditions that were prepared in advance and never mentioned, and because the arrangement is invisible, the effect looks like a property of the thing rather than of the staging. Nothing has to be faked for you to be badly misled.

When something is being shown to you, especially by someone with an interest in the outcome, ask what was set up beforehand and what happens under other conditions. Ask to see the case that did not go well. Anyone confident in the underlying thing will usually be willing to change one variable in front of you.