Long story short, this could have ended very poorly for a participant. It is never, ever appropriate to make a top rope anchor with only one non-locking carabiner attaching the rope to the anchor.
Petzl Dream in Vertical
Safety Stories — The Mountaineers
Climbing Anchors You Should Know
Lead Rope Soloing - filling the summer gap » Auftriib
Ice climbing Adirondacks: The Good, the Brittle and the Dirty
Rock Videos - AMGA
The extended rappel, explained — Alpine Savvy
How to Build Top Rope Anchors for Ice Climbing
Primer: Top-Rope Soloing - goEast
Safety Blog Posts — The Mountaineers
Lowering in Single Pitch and Multi-Pitch Climbing — The American Alpine Club