Few days ago, while making my ic burner, I notice one thing very interesting and weird. It's that AVR MCU ISP interface really doesn't need to be connected to ground at all. Why does AVR-ISP work without being connected to the ground?
I have tested this with few same package size avr MCU: namely the atmeaga328, 48v, 88v, 88pa and 168. They all work fine without connected to the GND.
My personal guess would be that it's probably because the avr is grounded via the MISO or the MOSI pins during the ISP session, since those 2 I/O pins can be programmed to connected to VCC or Gnd.