Add dummies ( <div class="dummy> ) after a loop to make formatting look good when using flex. Example - you have a layout with three tiles in a row but you have 4 tiles. Instead of the fourth tile on the second row stretching the full width and looking weird you add dummies and they will take care of it for you no matter how many are added.
.dummy {
margin-top: 0;
margin-bottom: 0;
visibility: hidden;
height: 0;
}
<div class="dummy"></div>
<div class="dummy"></div>
<div class="dummy"></div>
<div class="dummy"></div>