I've had the same when I was trying to use the url ([ url] [/ url]) tags in my own signature for the third time (two links are ok, three is a no go).
It's frustrating, but something that was already reported to Tom a long time ago.
I'll take advantage of this post to reiterate my proposal about accounts with more rep "earning" longer signatures.
That would allow to have several pictures/links in the signatures hopefully useful (in my sig I'd like to link to more tutorials/documentation and eventually doing some self promotion at time).