Created a custom dungeon, but I would imagine it could be duplicated in a standard campaign. My PCs are down a hall, the cleric had cast spiritual weapon on the far side of a room at the end of the hall and killed the last enemy in sight. Upon moving the spiritual weapon closer to the party, the spiritual weapon 'saw' yet another enemy, however since it and any of the PCs had no line of sight to each other (I assume that's why) the game started the combat music, everyone got into combat poses, however no initiative was rolled and the combat UI never surfaced. Only option is settings and quitting the game.
And naturally, it had to be at the very end of a 4-hour dungeon I had created with the last enemy standing between my party and that sweet sweet loot!