That's not a software bug, that's an image retention issue with your monitor. If it was that severe then it probably got into a state where it was sending severely invalid timings to the TFT LCD array, DC biasing it, which causes long-term retention and may even cause permanent damage if done for too long.

Software isn't supposed to be able to cause that. That's on your monitor.

That reminds me of the spookiest bug I've ever encountered: once, when resuming a Dell laptop from suspend at work, it showed a Windows desktop. Said laptop had been running Linux exclusively for several months (but it had previously been used with Windows). Interacting with the laptop made the expected xscreensaver unlock screen appear, and everything worked normally afterwards. The only explanation I could come up with was that, somehow, a snapshot of the Windows screen had survived intact in a corner of the framebuffer which the Linux driver didn't touch, even after months of power off/on and suspend/resume cycles, and a bizarre driver glitch made it visible in that particular resume cycle.