When RAW just doesn't give a plausible result, similar to Mercer's complaint. (That said, I think I could have justified the attempted action, as both a player and a DM. A level 20 hero knows a lot about how to use weapons to get the effect they want in combat.)
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."
Similarly, caltrops by RAW should slow oozes or swarms, which is hard to picture.
So I ruled that caltrops don't slow the armor and wouldn't slow other creatures where it was hard to make the logical case for it, even if, from a game standpoint, they would.