Whenever arow exceeds 12 columns in bootstrap the rest of the columns move to the next row, as if a next row is being created.
but according to bootstrap documentation you should always wrap the columns inside a row. so it is a bad practice.
see the effect in the example
<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<div row="container">
<div class="col-md-4 col-sm-6 portfolio-item" style="background-color:blue;">...</div>
<div class="col-md-4 col-sm-6 portfolio-item" style="background-color:black;">...</div>
<div class="col-md-4 col-sm-6 portfolio-item" style="background-color:green;">...</div>
<div class="col-md-4 col-sm-6 portfolio-item" style="background-color:brown;">...</div>
<div class="col-md-4 col-sm-6 portfolio-item" style="background-color:grey;">...</div>
<div class="col-md-4 col-sm-6 portfolio-item" style="background-color:yellow;">...</div>
</div>
<br><br>
<div class="col-md-12 col-xs-8 portfolio-item" style="background-color:blue;">...</div>
<div class="col-md-12 col-xs-4 portfolio-item" style="background-color:black;">...</div>
<div class="col-md-12 col-xs-8 portfolio-item" style="background-color:green;">...</div>
<div class="col-md-4 col-xs-8 portfolio-item" style="background-color:brown;">...</div>
<div class="col-md-4 col-xs-5 portfolio-item" style="background-color:grey;">...</div>
<div class="col-md-4 col-xs-4 portfolio-item" style="background-color:yellow;">...</div>
</div>