LVL2 Sivir had a Trap Claw and a Red Buff.
LVL3 Diana had Thornmail.
Diana uses her ability, procs Sivir's Trap Claw, but does not get stunned.
Sivir is stunned for the Trap Claw duration.
Checked the Diana players board after the round, but they had no units capable of causing stuns.
I suspect this is somehow caused by an interaction between Thornmail reflect and Trap Claws stun.