it seems there is possible an issue with tags right now where when you select a tag that has no menu item ( say a single tag on a article ) $active becomes NULL so everything gets confused.
I tried the "show tags" extension and core tags and both have the same result.  This was discovered using joostrap's pressmag template though the same thing happens with base v3.
http://screencast.com/t/tPvPEiLtUsMC shows the error being produced and  http://pastebin.com/pXKFtef8 shows $menu array after clicking.
| Category | ⇒ | Tags | 
 
                 
                Not sure actually... hmmm... i'll have to test this today.. The video shows how I did it, not sure exactly ( at this moment ) how to do the same test with clean install.
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                @N6REJ Has this been resolved by the PR as @Hackwar suggested. Please test and report back or this issue will be closed automatically in a few weeks
| Status | New | ⇒ | Information Required | 
 
                Closing as per statements above
| Status | Information Required | ⇒ | Closed | 
| Closed_Date | 0000-00-00 00:00:00 | ⇒ | 2015-04-22 23:59:56 | 
| Closed_By | ⇒ | brianteeman | 
Hi Troy,
Can you post step-by-step directions on how to replicate the issue on a clean install?