This happens very occasionally, although has happened twice today.
I'll go to http://stackoverflow.com then find a question with the c# and click it to view that tag's questions. However, the page i'm given shows questions tagged c and the URL has C# instead of C%23 in it.
I'm on chrome (latest version) and like i say it only happens occasionally. If I click back in the browser the urls look to be ok.
Please tell me there some ajax-type url encoding going on or something and I'm not going mad! The only thing I can think of happening is that I'm clicking the c# too fast, before URLs are sorted out or something.
Alternatively please tell microsoft to change the name of C# to something that doesn't need to be url encoded.