(I was thinking that it is similar to a corrupted wish but scenarios like this are also called something else. Like for when a good event changes into bad?)
A kid goes into a candy store that has "free" candy. (good)
But it turns out that the kid has really stumbled into an illusion of some sort. (not good)
Not only is the area not really a candy store but there is practically no candy at all. (bad)
It also turns out that the kid must find a hidden special piece of candy before leaving.
Otherwise the kid will instantly die as soon as they step foot back outside. (worse)