GLES2.0:

Some device will give a warning on compling shaders(yet the compling will succeed), and the rendering result is incorrect with blink & artifacts.

the problems is gradient calculation(interpolation) relies on the neighbor pixels,

if the neighbor pixels are not in the same conditional branch,  the gradient will be invalid and result may be undifed.

I guess it is an un-defined behavior, so some devices/drivers works good, some don‘t.

1 //like this
2 if( test )
3     color = texture2D(sampler,UV);
4 else
5    color = vec4(0,0,0,0);

One workaround is use mix() or step() instead of condition test. 

1 color = mix(vec4(0,0,0,0), texture2D(...), testVal);

When using condition branch, the texture sampling may be not actually executed, which will increase graphics performance.

One draw back on using mix/step is that the sampling is always performed, then interpolated/multiplied.