This is obviously to code purists -- which I am. IMHO, it's way better to know what you're doing. They certainly aren't causing any harm, but we all have seen things like:
<span class="title">Article title</span>
And the guy who did it says it's the same as using an <h1>, since the font size attribute is the same. If they don't understand the value of a semantic code, they will just see that the site is showing up more or less the same in the browsers and that's good for them.
Edit: It's back up now.
Mistakes? Hmm... Bad style, maybe, but hardly mistakes. They certainly aren't causing any harm.