Im gona start of by saying that im a noob, so it might be kind of obvious to some people (and i hope it is). I have been working on a robot model for the past 2 days now, and I finally finished tonight...or at least I thought I did. It turns out that when I switch from blender render to blender game and hit "P", parts of my model is missing, but when its in blender render everything looks fine. what happened?
Turn on two sided in the face menu.
most probably is a "wrong normals" problem, in other words, the model's face normals are pointing in the wrong direction!
Select all faces and do Ctrl/N keys to "Recalculate Normals outside"!