If you use abilities with a cast time in vehicles the default cast bar for wow will show up.
In addition the oUF_LS cast bar will also still be there.
Both of them work.

I saw this in the quest "The Talon King"
I do not know if this is reproducible.

I forgot to take a screenshot, sorry about that.

I will see if I can find another instance of this issue so I can post it here.

Fixed with https://github.com/ls-/oUF/commit/7aff711a9f7f027557b409259dd71019f89623cf

Castbar's a known oUF issue.. oUF-wow/oUF#220
However I'll check missing buttons issue, luckily I haven't quested through Sethekk yet :D

Alright, there was another vehicle section.
I managed to take a screenshot.
What is noteworthy is that this time I couldn't see the abilities I had and the addon cast bar was not there at all.

http://i.imgur.com/y8nV1mj.jpg

Edit: Here are two more screenshots of a third vehicle section, where it works as I described in the initial post.
http://i.imgur.com/vRUEUM2.jpg
http://i.imgur.com/TKRISaR.jpg
I hope this helps :)