WowAce

What steps will reproduce the problem?
1. Have a pet out (survival hunter, frost mage, and unholy death knight have all seen this error)
2. Enter a vehicle, especially one that has a "special" vehicle UI (the NPC vehicles from that quest chain in Jade Forest, stubborn weeds at Sunsong Ranch, the scorpions on Paragons of the Klaxxi)
3. Repeat if necessary (this bug is pretty erratic, I seem to encounter it every 20-30 vehicles)

What is the expected output? What do you see instead?
If you were in combat and the bug is triggered, your default action bars will "hiccup" and try to constantly slide off the screen to make space for the vehicle UI, which never appears.  If you were not in combat when the bug is triggered, your default action bars will stay in place, a "leave vehicle" button will appear next to your stance bar (or wherever it normally appears if you don't have a stance bar), but will be uninteractable.  Your default abilities won't be usable, nor will the expected vehicle hotkeys.  This will persist until your vehicle is killed or you reload your UI (in the event of an unkillable vehicle, such as the cannons in the Galakras encounter).  I have not found a way to consistently test the in-combat instance of this problem, but anecdotal experience suggestions that if you remain in combat without a UI reload, subsequent vehicles will also be "infected" (mutate on heroic paragons example here: bug occurs on Mutate 1 and you die because you cannot eat a parasite (hotkey 4 does not eat parasites as expected for the vehicle UI nor does it cast multi-shot), rezzing straight into Mutate 3 and the bug continues to occur).  Outside of combat, this issue does not appear to be more likely to occur back-to-back (testing on the SI:7 Report: Fire from the Sky vehicle quest in Jade Forest for alliance, instances of the issue do not occur one after another like they did on Paragons).

What version of the product are you using?
3.7.5 (release-860), as downloaded from Curse.  This appears to happen both with older variables/other addons and with a "fresh" install with only X-Perl (and Blizzard's own "addons") running.

Do you have an error log of what happened?
Yes and no; I have FrameXML logs from various instances of the issue here: http://us.battle.net/wow/en/forum/topic/13139343917?page=1 but there's no associated LUA error with the issue that I can see (I've never had an error pop up).

Please provide any additional information below.
Despite this being an issue with the default action bars, I believe it's something in X-Perl causing it, as about twenty minutes (in two ten minute bursts) without any addons being on was unable to cause the issue to occur, while I can usually get at least one instance of it with X-Perl on.   Additionally, all the characters who have run into this issue have been pet classes (two survival hunters, a frost mage, and an unholy death knight), and I was unable to get the issue to occur on the death knight after dismissing her pet.  Most of my focused testing (instead of just stumbling into it butt-first) was done on the Jade Forest/Pearlfin Village quest "SI:7 Report: Fire from the Sky" by repeatedly entering and leaving the NPC vehicle ("listening to the story") but I've also had the issue happen on Stubborn Weeds at Sunsong Ranch and in retrospect that's probably a better place to test (if only because the weeds don't pace back and forth and you don't hit two mini loading screens for each test attempt).  I'm not sure where the best place to test the in-combat variant would be (I've had it happen on cannons on Galakras and scorpions/mutate on Paragons of the Klaxxi, but neither of those are really great for solo testing).

Combat lockdown/FrameXML warning glitches, one of those unfix-able problems... FrameXML problems just unlikely to fix unless someone really know how to fix... :(

Lot of bugs like that. Noticed a bunch when you get mind-controlled during Garrosh

Oh blech :( Do you think it would be circumvented by having a bar addon as well? That seems... sort of round-about and super dangerous (and prone to proccing even more of them, even) but since it's the blizzard bars that are actually breaking? Man I dunno I REALLY don't want to go back to the default unit frames, but I was testing Shadowed for a few weeks there when I hadn't heard anything back and it has the same issue (less frequent so far but I leveled that death knight out of range of the quest I was testing on before so there's been a lot fewer instances for it to happen).

Actually I guess that's kind of not your department and I should just play with it. Thanks anyway :(