Strange moving icon on boot-up

We just purchased a used iMac with following specs: OSX 10.8.5, 2.93 GHz Intel Core i7, 16GB memory. After installing some software, I was rebooting system. When I did, a strange graphic (not the Apple startup graphic) came onto the screen (moving image). Hard to describe - looks sort of like the 'test pattern' graphic. Nothing booted up while it was showing. I had to force quit the system. I powered up again, and all was fine. What does this mean?