I attempted to make a simple cup for use in Tabletop Simulator.
When I load the .obj into the game, my model is missing an entire face (The bottom).
https://i.stack.imgur.com/5YtEB.png
However I've uploaded the obj to the A360 viewer by Autodesk and it looks fine.
I just started using Blender today so theres a good chance I made a simple mistake.