This is probably the most frustrating bug I have encountered in coda.
Normally when there is a syntax error in a formula, this happens:
However, when you make a syntax error inside of button label formula, an error is displayed and all your changes to the formula are discarded. (To recreate a syntax error just use List("A" "B").)
You forgot about a single comma? Too bad, now everything you coded is deleted and you have to start over (you can probably tell that this has happened to me way too many times).
What’s interesting is that, in case of a reference error, there is no error modal and the button is replaced with an action model.
While not ideal, it is significantly better than losing parts of your code.