Thief inside Wall
It happens all the time, when you use any kind of teleport ability, if you do that close to some textures like walls or rocks, you will end up inside the texture half of the time. However, if he literally WALKED into it, that’s strange, but if there is a chance that he used a teleport skill and ended up in the wall, that’s just a glitch.
In theory, there is no difference between theory and practice. But, in practice, there is.