Wiktionary's entry for left over reads:
Use left over after a verb, in a predicate phrase. When directly before a noun, use leftover.
Is this a general productive pattern? Otherwise, any reference for this usage note?
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A more common and productive pattern is for an open spelling in predicative position to correspond to a hyphenated spelling in attributive position: These shoes are worn out vs. worn-out shoes.
Alternatively, some writers may use the open spelling in both positions: "worn out shoes".
The use of closed spellings is not very predictable; it can be a matter of convention and might depend on the extent to which the specific construct has come to be thought of as a single unit/word.
The prevalence of the closed spelling leftover might be related to the use of leftovers or (the/a) leftover as a noun. Hyphenated left-over is an alternative for the noun as well as the attributive adjectival construct.
I can't think of another set of spellings for a participial construct with the exact same distribution as left over, left-over and leftover. Rundown exists as a closed spelling, but it seems to be found fairly frequently in predicative position.
Far away, far-away and faraway are not participial, but I think these spellings may show a similar distribution: far away dominates in predicative position (where "faraway" would look odd to me), but faraway can be found in attributive position, with hyphenated far-away as a variant to the closed spelling.1