debugWIRE
Atmel's debug protocol for small AVR chips, which borrows the reset pin and only gives it back if you ask nicely.
Small AVRs such as the ATtiny line and the ATmega328P have too few pins for JTAG. debugWIRE gets by with one wire: the reset pin becomes a half-duplex serial link offering breakpoints, single-stepping and memory access through tools like the AVR Dragon and Atmel-ICE. It is enabled with the DWEN fuse, which in turn can only be set over ordinary ISP.
The trap is that ISP stops working once debugWIRE is active, because ISP needs the reset pin. The way back is a dedicated debugWIRE command that temporarily releases it. Forget that, and the chip appears dead. On an Arduino Uno the auto-reset capacitor also interferes with the link, so a trace has to be cut. Its successor, UPDI, got a pin of its own.