What a cryptic error message, yeah? Good grief! Thanks Adobe! Googling didn’t come up with very much, either.

Turned out I was passing two constant registers to an operation.

So basically, in order to compare two temporary registers, you have to copy one of them to a temporary register first.

This code failed in my vertex shader:
ife vc7.x, vc5.x

But this code works:

mov vt1, vc7
ife vt1.x, vc5.x


Go figure. Wonder why that is. Solution found on a comment on this article.