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.

There is no limit to the number of activities in a work - in most works there are around two dozen built-in activities and perhaps one or two others - but only 25 can be simultaneously going on. In practice this is a generous limit and the most likely cause of breaking it is circularity mistake - where an activity is directly or indirectly duplicating itself.