For example:
If this is the HTML in the editor
<p><span style="font-size: 11pt; font-family: 'Times New Roman';">Test Test Test<br /></p>
selecting only a part of the element and clicking Strip Formatting will not remove formatting

Another example:
When in a list, individual items cannot be cleared of formatting, e.g.:
<ul>
<li><strong>bold</strong></li>
<li><span style="text-decoration: underline;">underline</span></li>
<li><em>italics</em></li>
<li><strong><em><span style="text-decoration: underline;">bold underline italics</span></em></strong></li>
<li><strong><em><span style="text-decoration: underline;">Underline, Bold, Italics</span></em></strong></li>
<li><strong><em><span style="text-decoration: underline;">Italics, Bold Underline</span></em></strong></li>
</ul>
selecting (a part of) a li element and clicking Strip Formatting will not remove the formatting.