When RAW just doesn't give a plausible result, similar to Mercer's complaint.
Most recent example: In a play by post game I'm running, the PCs scattered caltrops on the floor. An animated suit of armor came marching through to attack them. By the RAW, the caltrops should slow it, which -- to my mind -- requires it to be slowing down from pain. But animated suits of armor are basically crude magical robots. They will certainly get damaged by the caltrops (a little), but they're not smart enough to go "ow, this sucks, I'm going around a different way."