This issue is easiest explained by a screenshot:
The title of the thread linked to contains the
<input> starttag: Help on clear text in <input> !!.
I'm wondering what would happen if someone inserted an
<img> tag in a thread title...
Edit: Perhaps the solution is not to strip them, but to convert them so that they are not parsed as HTML.
Edit: This is reproducable by opening the thread I linked to while having the activity stream opened in another tab.