It was originally a meta-joke way of pointing out you'd left something out by intentionally leaving out the verb in the sentence where you pointed out you'd left something out.

It's since become a meme of sort as a more general way to flag as a joke that you've done something "accidentally" where a fully formed sentence would be more ambiguous.

E.g. "I accidentally created a scheme" might still not be entirely serious about it being an accident, but it's open to interpretation and sets a slightly more serious tone. "I accidentally a scheme" clearly signposts the title is at least in part a joke.