This has happened on a few of my plants, it doesn't seem to have any terminal effects (not to be dramatic ha) .. I always thought it was just because it got a little to cold to for 'em when we had the lights off at night, and they got confused. The plants that have that have stunted growth, but nothing else has happened, and I correlated it to the cold and it being to cold for them being to young, the older plants dont seem to get it.. these are my own assumptions based on no known facts hah.. hope it helps though
xox