Yeah, I figured that. That doesn’t make things any better, though – it just means that the “respite” after derailing is not a real buffer, but a false reassurance… 
Well, I’d reframe this… The “I’m never using nonzero pledges again and I realize that Beeminder will be less useful to me now?” was precisely the thing in space between “this complicated goal that I set up didn’t work like I thought it would in this unusual combination, whether because of a bug or my understanding or something” and “I’m ragequitting Beeminder now”…
The problem (as in “my subjective reaction”) is made much worse by the fact that this is the second time when Beeminder behaved very unexpectedly after derailing (and that was a completely unrelated issue!)…
It’s not that I have a problem with paying $5 – I don’t (unless it’s every day, which of course it’s not). I have a problem with Beeminder telling me “you’re safe” when in fact I’m not.
Again, sorry for my anger (also, I edited the initial post to make it sound less negative, after dreeves asked me to…), I’m better now, but the disappointment is still there…