Well, the times it happened the litterbox was pretty dirty, so maybe it is my fault xD
But I cleaned everything a couple days ago and now I am using a new and clean matras and it hasnt happened anymore.
I just bought Urine Off to spray over my old matras cuz that matras is softer ;p
I notied the same thing lately. The nails were split. Like, vertacilly.. Thought it was because Ihave a 2 dollar nailclipper from China. Never used it again and waiting till I find a good one..