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 special condition in R can only be used if R is a room or a region, and it test whether the current location of play is that room (or in that region). So it makes no sense for R to be some other kind of thing. This mistake tends to occur because the author assumes that something else is implicitly meant rather than the current location - for instance, one might write if in player, ... thinking that this meant some item currently under discussion: in which case the current run-time problem will occur with yourself as the thing which isn't a rule or region.