So you guys are most probably polishing your irrlicht 1.9 FVF pipeline, an I have some more glorious silent bugs and undocumented features of OpenGL
Each Vertex Attribute has to be aligned to its own type, a.k.a each first element of a vector of an attribute of any vertex needs to start at an offset which is a multiple of its own size
Sometimes it works fine, sometimes not... and when not its the alignment's fault
This is awesomely undocumented and you almost won't find it googling it!