I love it when people … [I accidentally posted this without finishing that sentence because I got distracted digging up an old gem: “mob justice”] … was about to say I love it when people pose these conundrums to the community!
My philosophy is to decide legitimacy not based on the strength of the excuse but on how cleanly you can formulate a general principle you’d be comfortable sticking to for all future cases. More about that in this blog post:
https://blog.beeminder.com/barfingcats/
(Hopefully it’s not too insulting that I’m sort of categorizing this as a Barfing Cats Excuse, namely, the kind of thing that feels like a fair excuse but ultimately the point of beeminding your goal was to make sure it would happen despite that kind of excuse. But if you can formulate a not-too-ad-hoc general principle – ideally with a nice bright line for applying it – then you could call it a non-legit derailment!)
PS: One more sort of relevant but embarrassingly self-serving (from Beeminder’s perspective) blog post is https://blog.beeminder.com/beenice which is like thinking of it as “I want to always stay on this yellow brick road unless something happens that makes it worth $5 (or $30 in you case – but you can cap the pledge at whatever makes sense) to derail”. That’s a nice bright line, and nice and unabusable. You just have to make sure the amount is something that will only feel tempting to pay in unusual circumstances.
Maybe that’s not even particularly self-serving, now that I think about it. You’re setting it up so Beeminder is providing highly valuable motivation in normal circumstances and getting paid for its trouble in unusual circumstances. As I’ve started saying recently, derailing is not failing!
Just step back occasionally and make sure the amount you pay Beeminder on average feels worth it for what you get out of it.