If you add a new interesting tag with a # in it, and then click on that tag, it searches for questions with the tag excluding the sharp (i.e. c# → c, f# → f). However, this only occurs if you click on the tag before reloading the page somehow. Refreshing, going to another page, whatever, and the tag works properly. The weird thing is, the url the tag points to is correct, but the results are wrong.
To reproduce, find a # tag that you don't have in your interesting tags list (or remove one) and add it, then immediately click on it.
Tested with the + (c++ tag specifically) symbol, but that seemed to work. There may be other symbols that cause this, but the tags weren't popular enough for me to find and test by flipping through the first few tag pages.