Wasn't progress or even PvE for that matter, but this happened in arena in Season 9: Playing 3v3 at almost 2700 MMR and we happen to (for god knows what reason...) come across a 1900 MMR team. Needless to say about 5 seconds after the gates open my WoW crashes and I come back too late and we end up losing the game and with it almost 100 MMR.