He didn’t sit on a mat unless it was straight.
This may sound like a rather pedantic point, but if a mat wasn’t straight it wasn’t in the correct position for sitting on. If someone still sat on said mat even though it was not set properly, it showed that they could easily slip into other bad habits and were prone to breaking other more important rules.
In the case of Confucius, of course, he punctiliously followed the conventions no matter how small and petty they may have seemed. If he broke those, how could he possibly adhere to the bigger principles that he preached?