The following example has been extracted from my automatically generated MP code.

\starttext

\startMPpage

string texstring;
texstring := "{\colored [r=0.00, g=0.00, b=0.00]{\definebodyfont [9.0pt][rm][default=Regular at 9.0pt]\switchtobodyfont [9.0pt]\setupinterlinespace [9.9pt]\tf E.g. publish/ subscribe}}";
path parpath;
parpath :=
(238,-167.00010681152344)..controls (238,-167.00010681152344) and (238,-167.00010681152344)
 ..(238,-167.00010681152344)..controls (238,-166.60010673470143) and (238,-166.20010665787942)
 ..(238,-165.80010658105741)..controls (238,-165.40010643392299) and (238,-165.00010628678857)
 ..(238,-164.60010613965414)..controls (238,-164.2001060472048) and (238,-163.80010595475545)
 ..(238,-163.40010586230611)..controls (238,-155.56353466513065) and (231.9683162849897,-149)
 ..(224.2552179837387,-149)..controls (223.85521783436539,-149) and (223.45521768499208,-149)
 ..(223.05521753561877,-149)..controls (222.65521740514737,-149) and (222.255217274676,-149)
 ..(221.8552171442046,-149)..controls (221.45521705001735,-149) and (221.0552169558301,-149)
 ..(220.65521686164286,-149)..controls (220.25521665589685,-149) and (219.85521645015083,-149)
 ..(219.45521624440482,-149)..controls (219.05521631807508,-149) and (218.65521639174534,-149)
 ..(218.2552164654156,-149)..controls (217.85521724037432,-149) and (217.45521801533303,-149)
 ..(217.05521879029175,-149)..controls (216.6552203077878,-149) and (216.25522182528385,-149)
 ..(215.85522334277991,-149)..controls (215.45522405083273,-149) and (215.05522475888554,-149)
 ..(214.65522546693836,-149)..controls (214.25522550518758,-149) and (213.8552255434368,-149)
 ..(213.45522558168602,-149)..controls (213.0552253720042,-149) and (212.65522516232238,-149)
 ..(212.25522495264056,-149)..controls (211.8552248680295,-149) and (211.45522478341843,-149)
 ..(211.05522469880736,-149)..controls (210.65522456100436,-149) and (210.25522442320135,-149)
 ..(209.85522428539835,-149)..controls (209.45522414149036,-149) and (209.05522399758237,-149)
 ..(208.65522385367439,-149)..controls (200.95716230583835,-149) and (195,-155.63452868682958)
 ..(195,-163.48966401016395)..controls (195,-163.88966411101083) and (195,-164.28966421185771)
 ..(195,-164.68966431270459)..controls (195,-165.08966445890775) and (195,-165.4896646051109)
 ..(195,-165.88966475131406)..controls (195,-166.25981210471718) and (195,-166.62995945812031)
 ..(195,-167.00010681152344);
picture pic;
pic := lmt_followtext [
  text = texstring,
  path = parpath,
  spread = false,
];


\stopMPpage
\stoptext

lmt_followtext crashes  on this code with:

metapost log    > ! angle(0,0) is taken as zero.
metapost log    > <for(1)> ...0)]shifted(-wid/2,0)rotated(angle(ad))
metapost log    >                                                   shifted.ap;draw.pic[(EXPR0...
metapost log    > <argument> ...caled0.5pt.withcolor.green;fi;endfor
metapost log    >                                                   ;if((autoscaleupfollowtext...
metapost log    > 
metapost log    > image->...ture;currentpicture:=nullpicture;(TEXT3)
metapost log    >                                                   ;currentpicture.if.str(SUF...
metapost log    > 
metapost log    > followtext->...b_n;setbounds.currentpicture.to.b;)
metapost log    >                                                   endgroup
metapost log    > <argument> ...parameter"path"),getparameter"text")
metapost log    >                                                   ;popparameters;
metapost log    > image->...ture;currentpicture:=nullpicture;(TEXT3)
metapost log    >                                                   ;currentpicture.if.str(SUF...
metapost log    > 
metapost log    > lmt_do_followtext->...meter"text");popparameters;)
metapost log    >                                                   endgroup
metapost log    > <scantokens> lmt_do_followtext
metapost log    >                               
metapost log    > <*> ...exstring, path = parpath, spread = false, ]
metapost log    >                                                   ; ;


When I adapt the parameters that create parpath in my code, other uses of lmt_followtext crash. The same combinations of text and path so far haven’t crashed basic followtext().

I’ve run into comparable situations in my own code where I had to add code to handle the existence of a (0,0) direction (e.g. when I had two identical points on a path, but that is not the case here). The above minimal example triggers the failure.

G