Varus Gameplay

I was in an ARAM a little while ago and the most curious thing happened while I was playing Varus. I ulted just before a death and witness the ability deal its damage and chain to additional targets (as it should). When I looked down at my UI I noticed it was showing my ultimate was not on cooldown and indeed when I pinged on it to relate the information to allies, it said it was ready but would not let me cast it. I'm pretty sure its just a visual bug in the UI but I figured I'd make the post just in case. This has only happened to me ONCE.