Shortly thereafter, I correctly interpreted this as the entire contents of Julie's stomach having decided to depart abruptly the way that it had come in. Oops.
The good news is that she managed to avoid fouling her own bed. The bad news is that Julie was in the top bunk and Katie was in the lower bunk, which is perpendicular to the top bunk, so her bedding got caught by the splatter.
I rousted Gretchen out of bed, then rousted Katie out of bed. Katie retreated to our bed while Gretchen started to get Julie cleaned up and taken care of and I started to clean up the room. Then Gretchen returned and we did more room cleaning, filling two laundry baskets with things that now urgently needed to be washed.
By 2:20 AM, the mess was contained, Julie was feeling somewhat better about it all, the bedding was changed, and both girls were sent back to their own beds. And then Gretchen went back to bed and I joined her.
Happily, Julie seems to be fine this morning.