tip burn. If you are still in veg and feeding you need to tone it down a bit and water for a day or two, start at half strength and move back up to where you want to be.
In a way I sort of like seeing a slight bloom of tip burn in late veg and late flower, it lets me know I am taking them right to their max intake point. Usually if I see it 2 weeks before my planned flip or harvest, at that point I just water and switch or flush and harvest.
Knowing the right colors helps you get a jump on other problems too. If its more pale yellow probalby more over watering, if its burnt yellow/rust/red its high nutes, if its brown with bubbles and crunchy its probably a ph issue....you just get an eye for it after a while...