One change in character completly changes the behaviour of this.

dlangSillyStringComparasonError.d

import std.string;
import std.stdio;

void main()
{

    string s ="string";
    string[4] c = ['a','g','t','s'];
    string[4] d = ["a","g","t","s"];

    for (int i = 0; i < c.length; i++)
    {
        writeln(c[i]);  //always prints 'agts'
        writeln(indexOf(s, c[i]));  //Always is -1
    }


    for (int i = 0; i < d.length; i++)
    {
        writeln(d[i]);  //always prints EITHER 'a','g','t' or 's'
        writeln(indexOf(s, d[i]));  
    }

    writeln(c); 
    // prints:
    // ["agts", "agts", "agts", "agts"]

    writeln(d);
    //prints
    // ["a","g","t","s"]

}