I've faced a question and failed to disprove it yet, so I am starting to think it is actually a proof.
Prove/disprove: It is given that in all MST's of a an undirected and weighted graph $G$, the vertex $v$ is a leaf. Then, in all such MST's, $v$ has the same predecessor (father).
False: Take the $3$-cycle on vertices $a,b,c$ with the weight of edges $(a,b)$ and $(a,c)$ equal to two, and the weight of edge $(b,c)$ equal to one. Then $v=a$ is the vertex that is a leaf on all/both MSTs, but it has different predecessors in each.