I recently used to it validate some php pages. Obviously, the validator only validated the X/HTML that was rendered, but it failed to pick up the following slips.
I had to edit what was a textarea, to make it a simple text input box:
<td class="hr"><textarea name="street_address_1" rows="4" cols="35" maxlength="100"><?php echo str_replace('"', '"', trim($row["street_address_1"])) ?></textarea></td>
<td class="hr"><input maxlength="100"
Even though I had failed to remove the </textarea> tag in my edited version, my page validated.
On top of this, I had a few instances of
<input type="checkbox" value="Yes" name="is_publisher">
The validator ignored the fact I hadn't closed the tag, and I'm pretty sure that's a basic requirement of XHTML 1.0 Strict Mark-up!?