An S-R latch (or S-R flip-flop) is a bistable multivibrator circuit made by cross-connecting two NAND gates.

S ---|     |
     |     |O-+------>Q
   +-|_____|  |
   |          |
              | |
   +----------+ |
   | ______     |
   +-|     |    |
     |     |O---+----Q'
R ---|_____|

If S=0 and R=1, the latch resets, and Q=0. If S=1 and R=0, the latch sets, and Q=1. If S=0 and R=0, the latch holds the present value of Q. In there cases Q' (Qnot) is the opposite of Q. However, if S and R are both 1, Q and Q' output the same value, meaning that Q = Q'. This is called the forbidden case.

If some poor EE student puts an S-R latch into the forbidden state, then Q=Q', and all the basic assumptions of logic fail. All the laws of mathematics and physics break down, 0 equals Infinity, atomic nucleii repel their electrons, matter ceases to exist, and the universe disappears in a puff of logic.