Scan chain
Wiring all of a chip's flip-flops into one long shift register used only during test. Without it a modern chip cannot be tested at all.
The problem is that a sequential circuit with hundreds of millions of internal states cannot be examined from outside — there is no combination of input signals that reaches into the middle. The solution is that every flip-flop gets a multiplexer on its input, and in test mode they are rewired into a chain. The tester shifts in an arbitrary state, pulses the clock once so the logic between the flops computes, and shifts the result out for comparison. The test vectors are generated automatically from the design. The cost is a few percent of area and a little extra delay on every flop. Access is normally via JTAG, which also makes the chain a known security concern — it can just as easily be used to read out secrets.