The povray documentation answers the question much precisely:

There are both advantages and disadvantages to using a non-zero flatness.
...
The biggest disadvantage is that if POV-Ray stops subdividing at a
particular level on one part of the patch and at a different level on an
adjacent part of the patch there is the potential for cracking. This is
typically visible as spots within the patch where you can see through. How
bad this appears depends very highly on the angle at which you are viewing
the patch.

  You can try setting the flatness smaller (or even 0) or increasing
u_steps and v_steps.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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