I found the following bug. 

I openen the file 10.txt from http://www.ohjelmointiputka.net/tiedostot/mpera.zip . Now Gedit seems to have problems with rendering that file. Also, gedit opens the file much slower than for example Leafpad 0.8.18.1. I tried to make a similar file and I was able to see it properly:

#include <stdio.h>

int main()
{
   FILE *fp;
   int a=0;
   fp = fopen("/tmp/test.txt", "w");
   for(a=0;a<100000;++a) {
     fprintf(fp, "fffg");
   }
   fclose(fp);
   return 0;
}