See what's happening here? The space-hyphen-solid decision is complicated by a coincidental word, and the difference is a lot more subtle, and likely to fool even experienced word people, than, say, bare foot (n.) vs. barefoot (adj., adv.).
A rundown, obviously, is a summary -- nothing to do with decrepitude. But a knock-down fight involves knockdowns. You might even dispute my contention that the solid form does not apply, but I think the juxtaposition with drag-out makes the truth clear.
Do any other examples come to mind? Heck the linguists probably even have a cute term for the phenomenon. If not, let's make one up.