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.

The ability to change map connections during play is quite constrained, and although the change ... exit of ... to ... phrase exists, it should only be used as a last resort. (Better to apply rules which cause the map not to be followed than to actually alter the map, is the idea.) In principle any exit which does not lead to a door can be changed, but here there seems not to have been exit to change.