I spent some time on IRC this morning trying to figure out why I couldn't add a CNAME to my domains. After some initial help and investigation, I found that, on 2 of the 3 computers tested with the same version of Chrome, a byte order mark was being appended to the "target" field of the POST data. This happened no matter what zone I tried to add a CNAME from. So for example, instead of getting "domains.tumblr.com", when I viewed the URL encoded POST data it turned out to be "domains.tumblr.com%EF%BB%BF". On the computer that worked correctly, this was not appended.

Pastebin of the conversation: http://pastebin.com/tifmjgGt

The BOM causing the problem is in the Tumblr instructions page at http://www.tumblr.com/docs/en/custom_domains, which I presume you copied your string from...?

<td>point CNAME record to "<strong>domains.tumblr.com</strong>&#65279;"</td>

Yep, that seems to be the case. I copy/pasted it the first time, then each subsequent try I just clicked the entry from the suggestions list for the text box. After typing it in manually it seems to work. The weird thing is that the error said "invalid hostname", when the hostname was actually fine, it was the "Alias to" field that was causing the issue.

I think that it's possible that the error message is not intended to refer to any specific field but just explain the type of error.
However, I do agree that there are some inconsistencies in the dns manager UI.