Most problems are detected when Inform translates the source text, but this is one of the exceptions, a "run-time problem" which emerges only during play.

This problem occurs if a phrase tries to move something which does not exist.

The most likely way for this to occur is if a plausible name has been given to what is, most of the time, indeed a genuine thing - but not always. For instance, if we say let the heroine be the tallest woman in the Amphitheatre, supposing that we have defined tall and made such a place, then we might expect that move the heroine to the Arena would always work. In fact it would fail if there were no women in the Amphitheatre at the time we tried to set heroine, and this is the problem message which would appear.