Yeah, it's common to have bad url or email address validation. Many sites do not accept short email addresses at all. As example a @bc.de (space is there intentionally). When it becomes a @ bc then it's interesting to see how many sites will fail with that.
I believe that TLDs with A records should resolve correctly if you add a period to the end, as in "http://ac.. As I recall, this came up when the "http://to. link shortener [no longer available] was discussed a while ago.
With the current trend of combining the address and search bar I doubt browsers will let you go to e.g. http://lol when you type "lol" in the bar.
In fact I just checked that typing "ac" in the Chrome omnibox makes a Google search for "ac" as I would expect. So the value of having an A record on a TLD is limited.
Edit: Somehow I missed the bar on top of the search results that says "Did you mean to go to http://ac " so there's some value after all.
