I have had that problem several times and in each case (and in the case of a friend of mine) all affected plants eventually hermied--two of them extremely close to harvest which bummed me out totally. Each time the amount of light was small (light thru a window and shadecloth one time) and happened more than once--like two times in a row or within a week of each other etc. I learned the hard way that keeping the light out during flowering was paramount (for me anyway) My hubby tends to forget to shut lights off so finally I took the light bulb out of the socket for the remainder of the grow. That solved the problem (tho we did end up with sore feet)