The truth is that it will delay the flowering cycle. When you switch the lights back to a flowering schedule the plants may need an extra few weeks to flower again. Worst case scenario is the plants begin to show signs of intersexing (Hermies). When plants are stressed it is a common occurance, and plants are senstive to light schedules. It probably wont hurt anything if its just for a day or two but if your in a hurry to get a harvest its not advised.