My plants are now over 1 foot high.
I did over water them last week, so I cut back the water, and gave them some water + fert last night.
Roughly around 1/4 litre per plant mixed with fert.
The leaves are green, no fert burn, they are I would think over stretched as they have poor lighting, but they are still growing, the stem is nice and thick, no really obvious problems.
It's happened to 2 plants now?!?

And it's not a stray seed problem.