1 Sallah: So what now, Monty?
2 Monty: We're completely at the mercy of the Nazis. I expect they'll execute us.
3 Prof. Jones: {sitting, with back to camera} Well that's just shocking!!
3 Monty: Yes dad, it is.
4 Prof. Jones: {turning around, holding the mystery novel up} The butler did it!

The phrase "The butler did it" is commonly credited to American mystery novelist Mary Roberts Rinehart, although she never actualy used it in her books. But she did have a butler do it in her 1930 murder mystery The Door.

The inimitable Cecil Adams of the Straight Dope has more, which I shan't repeat here.

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2015-10-14 Rerun commentary: Interestingly, TV Tropes lists several webcomics parodying or subverting "the butler did it", but not a single one playing it straight.

I think I have another trope to try get IWC listed on.

(If you see a new butler character introduced in the new strips, don't worry. He'll be perfectly innocent...)