Why does my iMac display show bands during shutdown?

Graphics glitches are sometimes resolved by resetting the SMC ( bit.ly/mfresetsmc) and NVRAM ( bit.ly/mfrstnv). If they persist or get worse, run Apple Diagnostics (June 2013 and later Macs, bit.ly/ mfapdg) or Apple Hardware Test (earlier, bit.ly/mfahwt), which can indicate a Mac’s graphics processor is starting to fail.