Not sure what could happen, but you could try instead on jq 1.9+:
$('#section-1.u2').finish().delay(200)
.queue( function(next){
$('.u2.b1').fadeOut(400);
$('.u2.b2').fadeIn(400);
next();
});
$('#section-1.u2').delay(400)
.queue( function(){
$('.u2.b2').fadeOut(400);
$('.u2.b3').fadeIn(400);
});