I went up to move Julie from the guest bedroom where she'd gone to sleep into her bunk in the room she shares with Katie. And when I opened Katie's door, I found the source of the "Thump!".
You see, Katie got a brand-new purple sleeping bag from Uncle Jeff and Aunt Carol tonight. And she wanted to sleep in it, so I'd put it on her mattress and zipped her up in it.
And some time later, she had slid off the bed with a loud "Thump!". Naturally, this put her into the only path that would allow me to put Julie into the top bunk. Oops.
I called Gretchen for assistance, handed her Julie, and picked up Katie and put her back in her bed, sans sleeping bag. And after Gretchen changed Julie, I put her into the top bunk.
Neither girl woke up during this operation.
I should sleep so soundly.