I've got an install of arch linux on my B+, but the test pattern is showing in the upper-right corner of the screen even well after the OS is fully booted. It disappears every now and then, but comes back when I take any action. I think it is getting re-drawn whenever there is activity on the GPU. I've looked through tons of documentation on config.txt hoping to find a solution, but to no avail. Has anyone else seen this issue? Is there anyway to get rid of it?
It sounds like one of two warnings.
- red square over temperature (>85 C)
- rainbow square power supply under voltage (<4.65V)
See http://www.raspberrypi.org/forums/viewtopic.php?f=29&t=82373 for fuller details.
I get the under-voltage (rainbow) when running quite a few of the full screen pi3d demos whenever there's quite a bit going on. The cpu monitor is down less than 10% and it seems to do it with or without the x server running. The power supply is a good quality USB and I've never had any other under-voltage problems.
However you can disable it if you really want (as per @joan's link)