You are right. I've fixed it in the manual and forgot to fix the tutorial description...
To be more specific: Originally (pre v0.10) if a bus had more than 1 non-Undefined inputs, the output returned an Error value (red wire). On v0.10 I removed all Errors from the build-in components (tri-state buffers with Undefined control pin and the bus), and I thought I should make it work like it's described in the tutorial (first non-Undefined input got forwarded to the output).
But, apparently, I never did the change, so if a bus gets more than 1 non-Undefined input, its output is Undefined.
I'll fix the description for the next release. Sorry for the confusion and thanks for reporting the bug!
PS: Actually I think a bus should behave more like an OR gate, which ignores all Undefined inputs. It should just OR together all defined inputs and return that as the result. But I think this might break some existing circuits, that's why I'm still keeping the current behavior. If you have any thoughts on the subject, I'll be glad to hear them.