I think there is a bug in 3.4. I just upgraded from 3.2. I have chosen to have the slideshow option enabled, but not to have it start automatically.
I tried the Themes that use icons for next, prev, start, stop (themes #2 and #9). When I click on an image on my page and it launches ColorBox, the control image is Stop (||) where it should be Start (>). However, the mouseover is the correct image.
Once I start the slideshow, the control correctly shows Stop but its mouseover is the wrong image.
This only occurs the first time I launch ColorBox for the page. If I close ColorBox and then relaunch it (without refreshing the page), the problem does not occur.
You can see this behavior at http://hand2mouse.com/visualcommunication/