I like the term barfing cat for any small to medium-sized phenomenon that threatens our goals or processes directly or indirectly. I think the category that we are discussing already excludes clear BS -- the idea is that you can really sympathize with someone who has to deal with an actual barfing cat. It's only when someone uses it as an excuse that it might morph into a weasel.
Putting aside whether it qualifies as a valid excuse, barfing cat —> ate junk food makes for an amusing but fictitious example, but I don't think it's so far fetched. It depends on all the other constraints someone might have in their life -- for example mass transit delays, family and work commitments, a grocery store that closes early, a rather broad definition of "junk food", and some food allergies. The barfing cat might just be the proverbial last straw from the narrative perspective when the rest of it were latent logistical or organizational issues.
There's an implicit part in committing to a goal or habit: when you say, for example, "I commit to learning N words in a new language every day," you also really want to be saying, "I want to get all my stuff together to make it possible and highly likely that I will learn an average of N words in a new language every day".
So that leads me into a related thing I've been thinking about for a while: for most of the challenges in my goals and habits, the larger difficulty lies in logistical rather than motivation. That's probably why, for a long time, I've been skeptical of emphasizing akrasia as the central issue. But it could be my experience is very different from others, and Beeminder already does a great job of countering akrasia once you understand how to set up your reminders and structure/quantify your goals in a meaningful way.
Regardless, I have mostly derailed on goals these days because of inadequate planning, inadequate systemization, overestimating my capability to cope with variations in logistical factors. Maybe there were some local moments in there we could call akrasia, but in the larger picture planning fallacy played a much larger role.
I think the commonly accepted advice to start goals really easy works well initially. Once things seem to be moving along for a while, you can get optimistic enough to want to increase the rate, or create more goals. Some folks, including me, have the inclination (vice) of starting goals/projects/making commitments in order to boost motivation (get the feeling that things are changing and so on.) But in general, multiple goals with overlapping constraints (unless you manage something like a waterfall) add some complexity. And I think it's easy to model resources like time and attention in terms of spatial analogues, rather than in terms of flow quantities (as Personal Kanban puts it).
I do think Beeminder already does a lot protect against some forms of planning fallacy by forcing things to be quantifiable, and structuring progress in terms of a daily average rate.
I think us talking generally about barfing cats (barfing cat events?) as things that happen is a good next step. Do you have a system or process underlying your goal? Does that system or process and all the other constraints (including other goals) leave enough room for a barfing cat to make its way through? How large of a barfing cat can it accommodate? I think it's good to have in the lexicon, right along with weasels.