Topping is done while plants are in Veg, usually in the beginning of Veg. If you are in flower and just topping or about to flower and thinking of topping you might have some issues. I don't think it will kill your plant but it will definitely put some unwanted stress on the plant at a time when it doesn't need it.
How tall is the plant? How many nodes does it have? You typically want to top around the 3rd or 4th node.