Two Guild mates go in to P2 at the same time, of any raid: Sion, 1st Tank, 1st Rancor. Doesn't matter. Let's say there's 5% left.
They each do the remaining 5% and end the phase on their runs.
They both get credit for 5% in the raid score list, however; the raid is still starting P3 with 100% remaining.
Not an issue in a regular raid where the roster refreshes each day, but on heroic, one of those Guild members just wasted an entire team doubling up 5% damage. Could have saved that team for the next phase. It's not so critical on Rancor or Tank for more advanced Guilds, and to some degree HSith, but it definitely matters to most guilds that are just starting or just able to clear HSith.