Upper left: image in question
Upper right: similar issue on POV sample scene with 'error_bound' = 0.1
Lower left: 'error_bound' = 1.8 will smooth artifacts but has some funny
self-illumination going on
Lower right: 'error_bound' = 0.01 will work OK, after many more hours spent
rendering...
(All three cases use 'count' = 800)

So it's about balancing 'error_bound' vs. render time.
Note, if you use textures (pigment and normal maps), radiosity artifacts can
even add to some nice result.

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> Upper left: image in question
> Upper right: similar issue on POV sample scene with 'error_bound' = 0.1
> Lower left: 'error_bound' = 1.8 will smooth artifacts but has some funny
> self-illumination going on
> Lower right: 'error_bound' = 0.01 will work OK, after many more hours spent
> rendering...
> (All three cases use 'count' = 800)
> 
> So it's about balancing 'error_bound' vs. render time.
> Note, if you use textures (pigment and normal maps), radiosity artifacts can
> even add to some nice result.
> 
> 
Things that can affect the quality when using radiosity :
pretrace_end : The default of 0.04 is usually to high for renders over 
800 pixels wide or high. Good values are under 0.01. 0.005 or 0.0025 are 
correct for renders size above 1200 pixels. More time in pretrace phase 
against less time of the render proper.

Reduce low_error_factor : Default to 0.5. Try values from 0.2 to 0.4. 
Will increase the sampling quality during the pretrace.

minimum_reuse : Default 0.015. Lowering can solve issues like your 
example using error_bound 1.8. A suggested starting value would be close 
to the pretrace_end value, maybe slightly lower.
pretrace_end 0.005
minimum_reuse 0.0045

Using the two values version of count : Very often, using count 100 1000 
will give less artifact.

Increasing nearest_count : Default is 5. Can go up to 20. Good at 
smoothing shadowed areas. Use nearest_count Value1 Value2 to enable 
adaptive pretrace.
Usage :
nearest_count 15 4

In scenes with small bright objects, you need a high count, but it 
greatly increase the rendering time. Use the importance mechanism.
Suggestion of use :
#declare HighCount = 10000;
#declare MeanCount = 75;
radiosity{ ...
    count HighCount
    ...
}
#default radiosity{importance MeanCount/HighCount}

And add :
radiosity{ importance 1 }
to your small but bright objects.
Larger bright object can get an importance between the set default and 
1. Range is 0..1.

This cause POV-Ray to only use a few samples for most of the scene, but, 
when sampling near the bright and important objects, to use more samples.

Whenever you have sharp inward angles, some samples can leek out on the 
back side of a surface, causing dark or bright splotches. Any 
displacement, however small, of the camera will cause those to change. 
Add some bevel or rounding will greatly help. Often done by adding a 
clipped cylinder tangent to both surfaces of a flat piece at an 
intermediate angle.
Some times, adding a little bit of focal blur can suppress those.

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