There is some problem with this quiz script. For some people it
doesn't work correctly. I changed it a little so that if people submit
more than once it still works, but still it doesn't work for some
people. what is the bug?

<!-- Begin
var ans = new Array;
var done = new Array;
var yourAns = new Array;
var explainAnswer = new Array;

var score = 0;
ans[1] = "c";
ans[2] = "a";
ans[3] = "c";
ans[4] = "c";
ans[5] = "d";
ans[6] = "c";
ans[7] = "b";
ans[8] = "a";
ans[9] = "d";
ans[10] = "b";
ans[11] = "b";
ans[12] = "a";
ans[13] = "d";
ans[14] = "c";
ans[15] = "d";
ans[16] = "a";
ans[17] = "a";
ans[18] = "b";
ans[19] = "c";
ans[20] = "d";

explainAnswer[1]="$$$";
....
explainAnswer[20]="$25";

function Engine(question, answer) {
yourAns[question]=answer;
}

function Score(){
var score = 0;
var answerText = "How did you do?\n------------------------------------\n";
for(i=1;i<=20;i++){
   answerText=answerText+"\nQuestion "+i+" ";
  if(ans[i]!=yourAns[i]){
    answerText=answerText+"The correct answer was "+ans[i]+":
"+explainAnswer[i]+"\n";
  }
  else{
    answerText=answerText+" Correct! \n";
    score++;
  }
}

answerText=answerText+"\n\nYour total score is : "+score+"\n";

//now score the user
answerText=answerText+"\nComment : ";
if(score<=10){
answerText=answerText+"You're completely unqualified";
}
if(score>=11 && score <=14){
answerText=answerText+"You really should study up";
}
if(score>=15 && score <=17){
answerText=answerText+"Not too bad";
}
if(score>16){
answerText=answerText+"You're a genius!";
}

alert(answerText);
}

//  End -->
</script>

You should make it clear that this is a JavaScript problem not a Java problem. 
- Post the context of the problem (i.e. full page)
- Explain exactly the error you are getting