Quick answer: Godot 4 save_resource crashing with stack overflow on circular references? Default save serializer recursively walks references - break cycles or use ResourceUID strings.
Item references Container references Item. Save crashes; project file is now corrupt.
Reference by UID string
Store res://path or uid://abc string; resolve at load. Breaks the cycle in serialization.
Or restructure ownership
One side owns; the other references by ID. Trees, not graphs.
Check before save
Recursive cycle detector before save_resource. Refuse to save corrupt structures.
“Serializers don't handle cycles by default. Cycles are a data structure choice.”
Most resource cycles come from convenience. The fix is restructuring; the engineering cost is one-time.