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.

Activities normally start, work through their business and end automatically, and for those activities this problem should never be seen. But it is possible to control their mechanism manually, so to speak, with phrases like begin the analysing activity with the pitchblende and end the analysing activity with the pitchblende.

Here an end seems to have been forced on an activity which was not, in fact, going on - this probably arises because of a mistake in running an activity manually.