Faced the Ghost gym on the Complex Gaming server, using a team that included a Shedinja with a Focus Sash. The rest of my team had fainted, but there were still several enemies left, each with Ghost moves, including some with Shadow Sneak.
However, each of them used a random move, as if they didn't have anything that could hit Shedinja. I suspect that it saw the calculation for damage saying even the super effective move would do 0 damage, and so picked a random move instead of the super effective one to break the sash.
This is the Pokemon, moves, and items on the Gym Leader's team. Shedinja killed all but Drifblim and Gengar, as they kept picking moves other than their ghost moves.