Hmmm, that's annoying.

I'm guessing, though, that because it's such a controversial issue, the test won't make it the sole determining factor. In this question, the subject-verb agreement makes it easy, as that's clearly a higher priority.

Great post, SoCan!!

The test is really well-written, so you're absolutely right that if there's some real gray area over whether an idiom is absolute or not it won't be the determining factor. In fact, many official questions use idioms as pure smokescreens to take your eye off of the real problem - something a bit more universal like subject-verb agreement or logical modifiers.

When you're going through SC questions, try to always see the big-picture in addition to (or maybe just instead of) the smaller-level idiomatic items. The GMAT is testing your problem-solving ability, not your memorization skills (and if it were to test memorization, wouldn't it seem prudent to give you a monster list of what you needed to memorize rather than leave us all to these forums to try to figure all that out?).