Now I'm not saying this is what happened to Marks cells, but just thought I would mention it.
Several times I have put cells on my smart tray, pressed the button, and one LED goes out straight away. I've removed the cells, squeezed in the tabs, tried again and that cell discharges like all the others.
Just a heads up while we're on the subject. I suppose if this happened to someone, they were using their smart tray to discharge quite a bit out of the cells and they didn't realise, then that cell would be overcharged.
G