It was when there were about ten people from another company in to do checking on the cutscenes.
The cutscene with Sae laughing in the Great Hall was still only half-finished, and the sound hadn't been properly added yet.
We replayed the bit around the time Sae is laughing so we could fine-tune the cuts, camera angle etc., and suddenly I heard a woman burst out laughing behind me.
The voice sounded a little younger than Sae's, but the laugh was full of unbearable mirth like Sae's was.
There was no sound, but there had been a sound then, so the voice being checked was stopped.
"...Someone... was laughing just now, right...?" I asked timidly, and the staff began to answer with abstracted faces.
"I didn't hear anything."
"U...uhh, well. It came from the TV."
"Huh, didn't it come from outside?"
"Well then, would anyone who heard it please point in the direction it came from?"
All at once, everyone pointed together, and I figured out the source of the voice.
The voice seemed to somehow have come from behind my right shoulder.
In other words, putting the story together, someone invisible standing behind me had suddenly begun to laugh...
The staff also said, surprised, "It's the first time anything like this has happened, but it did, didn't it?"
(Later on)
Some time after this incident, we had a teleconference with Nintendo.
During the meeting there was a mad-sounding laugh coming from the window, and the meeting was temporarily suspended.
The laughter was closer than it had been when I'd heard it before.
Someone on the other side of the TV said, "Perhaps someone was walking past and laughing?" but the meeting room was on the sixth floor.