Poppy ult sometimes doesn't fully knock back Darius
Sometimes, when Poppy ults Darius, he only partially goes back. I could only test it on bots, so I couldn't get it consistent, but I think it's when he starts his Q at some specific time shortly before you hit him.
Update: Could only get it to work on Darius bot after trying with Veilix and a Shyvana bot. Here's a picture of places I've seen it happen and a video of it happening. Doesn't seem to be consistent, though. The very first time it happened, I hit Renekton bot and Darius bot with the same ult, but only Darius bot stopped like this.
The spots may not be totally accurate, since I'm going by memory of where they are (except the one in the video). However, I do remember the top lane circle is the first time it happened (with the Renekton bot also hit).