From a commercial for Mr. Clean (a household cleaning agent), seen recently on tv:
(1) He’s cleaning things that we don’t even know what they are.
The relative clause, boldfaced above, has no gap of “extraction” in it; instead, the pronoun they is anaphoric to the head of the relative, things. The gapped version is stunningly worse:
(2) He’s cleaning things that we don’t even know what ___ are.
In (2) the gap is inside an “anaphoric island”, a WH clause, and worse, it’s a subject gap, so using a “resumptive pronoun” instead of a gap, as in (1), repairs the problem in processing the relative clause — yielding something that’s not standard English but is comprehensible.
So (1) is an example of what I called in a Language Log posting from three years ago a ResIsland gapless relative (with a resumptive pronoun repairing an island violation). They are pretty common, and many of them have a somewhat vernacular and playful feel to them, an effect that might make the Mr. Clean commercial noticeable and memorable.