The ghost only the phone could see

For days I kept seeing ghosting on the party as they walked: a faint double of a follower, just behind them. Every screen recording came back clean. The fix only started when I filmed the monitor with my phone, and that clip caught it: a follower doubled mid-step.

The cause was a sampling beat. The party leader drops a trail breadcrumb every 0.08 units, which at walking pace is about every second frame. The followers measured their position from the last breadcrumb. So on odd frames they stood still, and on even frames they jumped double, while the leader and the camera glided smoothly between them.

That also explained the clean recordings. A 30 fps capture always landed on the same half of each pair of frames, so it never saw the stutter. My eyes, watching at 60 Hz, blended both halves into a ghost.

Followers now measure from where the leader actually stands, and a per-frame position log shows all three advancing on every frame. Two earlier suspects were removed along the way: see-through copies of the party, drawn so they could be seen behind walls, which painted pale doubles onto the entrance posts. The fix was deployed the same hour.

The next morning the party stopped walking in single file. Each follower steps about a third of a cell to its own side of the trail and eases back in where a passage narrows. After the ghost, that change was checked for flicker: across 266 sampled frames, each follower changed facing only one to four times.