Wait a minute, I take that back. I left IE open long enough for the letter to reach the limit (3450), and then I got a pop-up error. The same with FF.

The reason for that is that when letter = 3449,
else if( letter < 3450 ){...}

is true and the content inside {...} is executed. That contents increments letter to 3450. So the next time that timeout calls alterText(), it attempts to execute this first:
underscore = document.getElementById("n" + letter).innerHTML;

since letter is now 3450 an such id does not exist, it throws a runtime error! You must check for the limit first.

Here's the revised code: