One common mistake that I've found in many web pages is when a nested list is needed. Usually, we think that the following code is correct:
<ul class="list">
<li>A</li>
<ul class="sublist">
<li>1</li>
<li>2</li>
<li>3</li>
</ul>
</ul>
Producing this:
- A
- 1
- 2
- 3