On July 25, a Twitter user noticed something strange. The site has long suffered from joke theft—usually involving spam accounts that copy and paste other user’s popular tweets in a cheap bid for attention. But it seemed like, for the first time, one comedian’s stolen jokes were being quietly deleted and replaced by a statement from Twitter referring to “a report from the copyright holder.” After years of flagrant theft, the new policy was heartening. But that the network took so long to acknowledge that a verbatim retelling of a joke by another user counted as copyright infringement points to the issue’s larger nebulousness, which has plagued comedy for decades. If the wording of two jokes aren’t exactly alike, how can you tell one was stolen?

Twitter’s crackdown on joke-stealing came at the same time that a lawsuit was filed against Conan O’Brien’s late-night show Conan by a relatively unknown comic who said the show had ripped off his material. Robert Kaseberg, who has contributed jokes to The Tonight Show, said the show had lifted four of his jokes from the Internet, pointing to very similar wording in Conan’s monologue. The jokes are undoubtedly similar, but could just as easily be a symptom of parallel thinking—after all, there are only so many easy punchlines out there for every news story. Or, as the Conan star Andy Richter quickly tweeted, “There's no possible way more than one person could have concurrently had these same species-elevating insights! THESE TAKES ARE TOO HOT!” When it comes to allegations of theft, one standard has always held in comedy: You had better be claiming ownership of some truly original material.

Late-night shows are the easiest targets of such complaints, since they’re designed to quickly react to the news with punchy one-liners, a mantle since taken up by many a Twitter user. Saturday Night Live once featured a parody of Brett Favre’s Wrangler jeans commercials after his sexting scandal; people pointed out other comedians had made the same joke online. Sure—because it was the most obvious thing to do. Aiming for the lowest common denominator doesn’t equal plagiarism. Still, complaints about theft of someone’s premise remain a frequent issue in the comedy scene. As Splitsider’s Adam Frucci once noted, if you want to accuse someone of stealing your joke, make sure it wasn’t only highly original, but also highly similar, and visible enough that someone famous might have seen or heard it.

Often, allegations of joke-stealing don’t stick. A Cleveland sketch troupe complained about a bit featured on Conan in 2011, saying it was inspired by their stage act, but failed to realize the comedians performing it on the air had been doing it for years. A Saturday Night Live sketch that featured actors wearing tiny hats seemed superficially similar to a sketch that had aired a few years before on Adult Swim’s Tim & Eric Awesome Show, Great Job!, although outside of the tiny hats, their scripts had nothing in common. While it’s possible the writer had seen the Tim & Eric episode and had gotten that one particular element lodged in his or her subconscious somewhere, it’s a leap from that to outright plagiarism.

As with TV, when it comes to stand-up comedy, it can be hard to find the line between parallel thinking and insidious theft. Premise, as always, remains key. Louis C.K. famously nursed a grudge for years against Dane Cook (back in the day when Dane Cook was a far bigger audience draw in stand-up comedy) for telling jokes that had a similar core concept to his material, although the delivery was very different. Both C.K. and Cook had jokes about giving their kids a strange name, but C.K. took it in a surreal direction (he wanted to name his child “Ladies and Gentleman,” so he could spontaneously cry out “Ladies and Gentleman, please!”) compared to Cook’s more obvious approach (that it’d be weird to name your kid Optimus Prime). One similar premise might have been forgivable, but there was enough crossover to sow discord for years, until the two hashed it out face-to-face in a fascinating second-season episode of Louie.

The idea of a joke as intellectual property hasn’t always been ironclad. Many stand-up comedians used to rely on writers for their gags, reworking the material they were given to fit their style, though that practice has since fallen out of favor. Joke thievery was an issue to be worked out behind the scenes, often with threats of physical violence. David Brenner notoriously approached Robin Williams’ agent and said, “If he ever takes one more line from me, I’ll rip his leg off and shove it up his ass,” an incident reported on years later in Richard Zoglin’s book Comedy at the Edge. Williams frankly discussed his penchant (as a younger man) for joke-stealing on his fantastic appearance on Marc Maron’s podcast in 2010, where he admitted to not really understanding, at the time, the weight of what he was doing. Part of the problem is, after all, the ephemeral quality of stand-up comedy—while performances are honed to perfection, some tossed-off gags at an open-mic night might never be heard from the same comic again.

Though the subject of joke-stealing has prompted debate for decades, the Internet has recently made it easier to point fingers. In some cases that’s a good thing: Carlos Mencia’s penchant for reworking other people’s jokes was exposed online and then discussed on Maron’s podcast, where the comedian finally admitted to the practice after Maron pushed him on the issue (at the behest of many in the stand-up community). A South Park spoof of the film Inception was found to be so similar to a CollegeHumor video that the show’s writers eventually admitted to being inspired by it.

But these exceptions are notable. Even with Twitter finally acting on copyright complaints and erasing examples of word-for-word theft, there still won’t be too many cases of joke-stealing that come across as being so clear-cut. As much as it sounds like an easy excuse, two similar punchlines may just be a coincidence. When you hear about the Washington Monument being 10 inches shorter than its reported height, it’s hard not to think of a penis joke, however dumb. Not every gag can be a winner.