{GAMEPLAY} ~~ Nocturnes Duskbringer.
It's only a minor (not game-breaking) bug.
Nocturnes Q appears to "flash" when on cool-down, while nocturne is auto-attacking. I'm assuming it doesn't actually reduce the Cool-down when attacking, but it can be rather off-putting.
Reproduction :
Play Nocturne. Take Q, Duskbringer. Q any enemy or neutral minion. Auto-attack.