In the "font-emoticons.php" all the smileys are applied on a span tag. The problem is that the author think the span tag can be self-closing. That is a flaw that seldom is noticed, since most browsers correct the mistake in certain occasions.
But the problem becomes very much evident and troublesome once you try to style the smiley with css, let's say make the size 80% of the surrounding text and in another color. Then, the styling get applied not only on the smiley, but on everything else after it.
So to close the span properly, on line 36 change this:
$code = '\\1<span class="icon-emo-'.$this->name.'"/>\\2';
$code = '\\1<span class="icon-emo-'.$this->name.'"></span>\\2';