I'm running an iMac4,7 and a Mini, both OSX 10.7.5. POVRay 3.7.0RC7. No
regression tests were performed on earlier versions or RCs.

Two items of interest. Labed A) and B)

A) The spline series is not required to be only ascending, as the following
works:
Results in Hard crash.

#declare Good_Spline =
 spline { natural_spline
  1.25, <-0, 20, -16>,// <--- the significance is here
  1.00, <-0, 24, -12>, // start
  0.88, <-12, 32, 5>,
  0.75, <-3, 30, 7>,
  0.63, < 5, 32, 7>,
  0.50, <8, 33, 3>,
  0.25, <10, 34, -1>,
  0.13, <-5, 32, -3>,
  0.00, <0, 33, -0>, // end
  -0.25, <-0, 28, -16>
 };

But discontinuity in the first column this structure causes a crash:
#declare Bad_Spline =
 spline { natural_spline
  -1.25, <-0, 20, -16>,// <--- the significance is here
  1.00, <-0, 24, -12>, // start
  ...
  0.00, <0, 33, -0>, // end
  -0.25, <-0, 28, -16>
 };


B) Parser displays this possible error around the vnormalize() function in
Spline_Trans()
   Possible Parse Error: Normalizing zero-length vector.
See code snips and #debug trace--- I can't see where the code would be
normalizing a zero-length vector.
I got reasonable and acceptable results.

1) Thrown when the first column of the first entry is <= -0.10
2) Thrown when the last column of the last entry is < 1.10

#declare Bad_Spline =
spline { natural_spline
 -0.05, <9.00, 5.6-2, -11.755>, <-- 1) in example below
 0.00, <9.00, 5.6, -11.755>, // start at amp
 0.25, < 6.0, 6.41, -10.8>,
 0.50, < -2.5, 5.4, -11.5>,
 0.75, <-10.0, 6.3, -12.0>,
 0.94, <-16.0, 5.7, -12.5>,
 1.00, <-17.00, 5.4, -12.175>, // end at tank
 1.05, <-17.00, 5.4-2, -12.175> <-- 2)
};

#declare NrSamples = 100;
#for( Nr, 0, 1, 1/NrSamples )
 #debug concat ( "Loc=", VStr(<0,0,0>+Spline(Nr) )," LocNext=",
VStr(<0,0,0>+Spline(Nr+0.1)), "\n" )
 Spline_Trans( Bad_Spline, Nr, y, 0.1, 0.5 )
#end

Results:
Loc=<9.000000,5.600000,-11.755000> LocNext=<8.526543,7.326084,-11.408949>
Possible Parse Error: Normalizing zero-length vector.
Loc=<8.986093,5.917270,-11.741055> LocNext=<8.433156,7.352713,-11.358516>
Loc=<8.965508,6.197908,-11.721148> LocNext=<8.330271,7.359527,-11.307260>
Loc=<8.937962,6.443597,-11.695794> LocNext=<8.217604,7.348206,-11.255697>

#declare Good_Spline =
spline { natural_spline
 -0.1001, <9.00, 5.6-2, -11.755>, <--  in example below
 0.00, <9.00, 5.6, -11.755>, // start at amp
 0.25, < 6.0, 6.41, -10.8>,
 0.50, < -2.5, 5.4, -11.5>,
 0.75, <-10.0, 6.3, -12.0>,
 0.94, <-16.0, 5.7, -12.5>,
 1.00, <-17.00, 5.4, -12.175>, // end at tank
 1.05, <-17.00, 5.4-2, -12.175>
};

Results:
Loc=<9.000000,5.600000,-11.755000> LocNext=<8.495552,6.559671,-11.377576>
Loc=<8.979286,5.748926,-11.734164> LocNext=<8.402657,6.598457,-11.327640>
Loc=<8.952854,5.884985,-11.708339> LocNext=<8.300710,6.628493,-11.277335>
Loc=<8.920372,6.008590,-11.677987> LocNext=<8.189379,6.650191,-11.227123>

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