I was working on a session at an indoor wall. Whilst instructing a group of children I saw a person set off leading. The belayer had a lot of slack out and I recall thinking they would struggle to take in the slakc quickly once the first bolt was clipped.
The belayer did struggle but much more than expected. After a few seconds they hadn't taken in any slack and seemed to be fiddling with something. I couldn't see but expected they had put a grigri on the rope upside down and were taking it off to sort it out. The climber was still going.
I was backing up a child and had the climber lowered off as quickly and safely as possible. I rushed over to the struggling belayer who was just doing up the screw gate again. The climber had been at least at 4th not attached to the rope at all. They had no idea they had been off belay.