Hey, this is the forst time ive had this happen to me. It happened when i tried to make the border edges thicker, im not sure if thats what caused it, because i then restored the defaults and it still happened.
As soon is a try to extrude a border edge and move the edge, some of the polys just disapear, but the mesh stays.
Ive tried for a while, but i cant find out why, it was fine until the guy in the car tutorial showed me how to make border edges thicker.
Heres a picture to show you what i mean.