I ran into an interview two days ago and came across one strange definition of safe edge.
We are given an undirected weighted Graph $G = (V,E)$ with all distinct edge weights. Assume that the graph is connected.
Safe Edge Definition: if an edge $e \in E$ is not contained in any cycle, we called it a safe edge
Following is a theorem based on the above definition.
Theorem: All the safe edges must be in Minimum Spanning Tree (MST).
Please compare the definition of a "safe edge" on this interview with the classical definition of safe edge at all courses like here: MAIN SAFE EDGE THEOREM.