I created a fresh Joomla installation, and created an article with a tag.
I turned Error Reporting to Maximum. When I click on the tag I don't see any error.

Is that how I should reproduce this issue?

Hi Bjorn,

The issue is visible on the feed page of a tag. So if you are on http://localhost/tag/1-some-tag go to http://localhost/tag/1-some-tag?format=feed&type=rss

Thanks for testing!

Turned Error Reporting on Maximum (and later Development) and then I went to: http://localhost:8888/jab/index.php/component/tags/tag/2-tag1?format=feed&type=rss

But I do not see any error, I also checked with Chrome's console and Firebug's console and the Error console of Firefox, but there were no errors.

Could you please check your PHP error log file? I don't get it shown in the browser either (even though I use E_ALL, PHP 5.5.11), but it's all there in the error log, I just tried again now with the latest Joomla

I can confirm the php notice with J3.3.1
[23-Jul-2014 18:03:39 Europe/Berlin] PHP Notice: Undefined property: stdClass::$title in /Applications/MAMP/htdocs/test/Joomla_331-Stable-Full_Package/components/com_tags/views/tag/view.feed.php on line 67

I can confirm that the PR resolves the php notice

In fact I get 2 errors here:
the same as Brian above
+
PHP Notice: Array to string conversion in /Applications/MAMP/htdocs/trunkgitnew/libraries/joomla/filter/input.php on line 202

This PR solves the first one indeed.