This isn't a 1.1 bug, I've seen it before on 1.0 and I think on one or more of the beta releases before that.

Sometimes (rarely), when I select a spell on the action bar, the border around the spell becomes red, and I can't cast it.  Right clicking on the map will usually deselect the spell and clear the red border, but when I try to reselect the spell the same thing happens.  If I bring up the spell list instead, I can successfully select and cast the spell, so it's not a matter of not having enough mana or the spell still being in cooldown.

Here's a pic of how the action bar looks:

At the time I took that screenshot, I was trying to cast Chaos and was unable to, until I pulled down the spell list and selected it from there.