Clear item from Chrome address bar autocomplete
Solution 1:
I don't think you mistyped c#, I think this was a bug that was introduced recently. As I also just started getting this behaviour.
See https://code.google.com/p/chromium/issues/detail?id=406029
I'm guessing that as #'s are used in url's more often now, chrome decides that anything with a # behind it should be considered a web address. For example if you type: "wdqwuuw#" it will auto-correct to "wdqwuuw/#", which you of course don't want.
Solution 2:
Highlighting the entry+pressing Delete should work, but anyway an alternative would be to selectively delete the history instead:
Type chrome:history into the address bar.
Search for C/# in the search bar(top right corner).
Check all C/# entries and click Remove selected items.
Confirm removing items.
So next time you type C into the address bar you would get C#!
EDIT
I'm also using Chrome v37 (latest) and the method in your answer works for me?
I type C/ so that the incorrect one(C/#) shows up.
I move my mouse over the autocomplete so it is highlighted as in your picture.
I press Del- it gets deleted!
I can't think of any reason why it doesn't work for you though. Sorry I can't be of more help :(