Typically a blocked hot-shot injector due carbon build up. No lightup - Fuel flow goes in unburnt and gives the smoke until pressure ratio hits limit and reheat cancels.
Exactly.
The reheat goes through all the light up procedure, even if it doesn't light, until the pressure ratio across the turbine reaches a set level monitored by the P2/P4 Fluidic Flip-Flop (which corresponds to a certain nozzle position) when the system automatically cancels reheat and puts the nozzle back to the dry position. That way the engine will give the best thrust it can rather than the reduced thrust because of the open nozzle.
The cure used to be to clean the hot-shot injector in-situ by rotating an old speedo cable in the injector fed along the feed pipe - it's very well hidden when it's in place.