If a player dies/gets killed with a ignited smoke in his inventory it smokes forever.
Proof -
The Kill : (05:12:10) http://www.twitch.tv/break71/v/24474693
Inventory confirmation : (05:21:20) http://www.twitch.tv/break71/v/24474693
(The ruined smoke grenade in his chest rig is causing the infinite smoke and seems to be bugged in the slot)
Infinite smoke in action: (05:24:05) http://www.twitch.tv/break71/v/24474693