Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
A builtin wrote an effect word that its axis does not recognize, or it listed the wrong number of words. These are the same mistake seen from two angles. The effect line is positional. The first word answers the first question, the second word the second, and so on. So if you write too few words, everything after the gap slides into the wrong slot. And if you write a word that the axis does not know, it is an answer to a question nobody asked. The engine does not round it to the nearest option, because a near miss is still a wrong answer. Picture a form with fixed boxes: write an unknown word in a box and the clerk cannot process the form. The message names the builtin and the word. The axis itself is printed on a separate line, birth.effect_axis, so you can see which question was being answered. Fix it by using a declared word, or add a new word to effect.feml. One more thing: an effect is written once at birth, like a kind, so a second effect declaration for the same being is refused too, and counted.
Text from the Rush engine's error catalogue.