https://bugs.freedesktop.org/show_bug.cgi?id=101596
--- Comment #4 from Matias N. Goldberg dark_sylinc@yahoo.com.ar --- +1 I get this error too.
This bug is NOT https://bugs.freedesktop.org/show_bug.cgi?id=97059 which gets fixed by either using DRI2 or selecting Triple Buffer in Blender.
This error started after I pulled latest git; and a faster way to reproduce it is to simply click on the Materials pane; where automatically everything starts going wrong within Blender.
I tried debugging the problem but came out short. My guess was either in the vertex shader's key.clamp_color would become true for the same variant where it used to be false, or that the same would happen in the pixel shader with clamp_color/persample_shading/ati_fs/external (i.e. test if any of st_fp_variant_key members changed but the same variant was being used).
However all shaders appear to be using consistent settings, which leaves me puzzled.
As Sebastian Parborg, undoing the changes "st_update_fp" fixes the problem.