In the room on round 5, before you go out, if someone lags out they cannot join back or someone else cannot join in. It just says they are too close to finishing it so yo u either have to two man it, which is near impossible, or just leave it. Once you beat the boss you can get the rewards but even then you have to go down to the chest room so if you lag out then, it'd probably do the same. Bungie just needs to fix it. Like, why can people join as Atheon/Crota/Nightfall Boss die and still get loot, yet you can't get someone back when it is needed most. I hope they fix it soon.