hey at first I thought it was burnt too,,,as in light burn,,but as max316420 is asking and from your snap are you growing in that green house? cause if so I take it the plants not under a lamp of somkind? it dose look like that affected area is in a hotspot from a lamp, and thats why its only that small area. But if your not under a light, then it could just be after the topping. I nip back all my plants and on occassion its diffusing of hormones can result in a small area of mutational growth. Growth that just dosnt work. I doubt its a PH inbalance as it wouldnt just be the one area, you would see other leaves affected.
So either light burn, or you havnt spilt anything on it? maybe a splahing of undiluted liquid fert? Personally I wouldnt worry to much untill there was 'damage' like that over two or three areas,but checking your pH wont hurt. Whats your medium?
take it easy.