grab (catch / take) the tiger by the tail

To grab, catch or take the tiger by the tail is the idiom which refers to the fact that if you grab the tail of a tiger, you will eventually have to let it go. It is another way of saying you should be wary of your actions due to the undeniable consequences. Literally speaking, if you grabbed the tail of a tiger, it may stop him in its tracks, but eventually you have to let it go and deal with the tiger being angry at you.