Blender 2.8 faces not visible I'm following this tutorial:

https://www.youtube.com/watch?v=WxMwa0njGSM

on modeling. At 18:52 before adding a subdivision surface modifier my object looks like:



After subsurf:



It looks like two overlapping planes but I don't see the vertices that create them (I've already cleaned 
up double vertices). 

How can I remove that face so that I don't get that wedge after subsurf?
Probably a normal issue. Select the entire mesh with A and then press Shift N to recalculate the normals.