If you ever spot Kieran Culkin looking shifty while lurking near the prop section, you really ought to raise the alarm.
The US actor, 42, recently confessed that he has got a track record for causing a bit of trouble when he finds himself at a loose end backstage.
So if you see something, say something - as you never know, he could be up to his old tricks again and be plotting a similar prank to the one he played on Mark Ruffalo in 2000.
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And that ended up with the Shutter Island star, 57, getting high as a kite on the opening night of an off-broadway production.
Take a look at what Ruffalo had to say about being the victim of Culkin's high-stakes bait and switch here:
Back in 2012, Ruffalo revealed that a 'naughty young actor' - AKA Culkin - had pranked him on the opening night of James Lapine’s play, The Moment When.
During an appearance on The Graham Norton Show, the Wisconsin-born star explained: "I smoked a joint in the first scene.
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"Of course, there was a very naughty young actor I was in the play with, who on the opening night with all the critics, he slipped a real joint onto the prop table.
"Now when you're acting, you overdo everything," he told Norton.
"When you're out at a party, you take two puffs and say, 'That's enough, I'm so stoned'.
"But I was acting," he laughed, before explaining that he was taking elongated drags from what he thought was a fake joint.
"Between the two of us, we smoked a giant blunt on stage. And I'm like, 'Man, I am in it tonight, I am feeling it, I'm on fire man'!"
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As the effects of the marijuana began to take hold, Ruffalo explained that he then came to a sobering realisation about what was going on after spotting Culkin chuckling backstage.
"And I just went...'Oh, sh*t," he added.
Although you might be imagining that this resulted in a baked Ruffalo rambling through his lines in front of the audience, he reckons it actually improved his performance.
"At the end of the play, I got the best reviews of my entire career," he laughed. "But I’d never do that again."
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More than a decade after Ruffalo shared the comical story with the world, Culkin finally decided to fess up to the prank publicly.
In a recent interview with The Guardian, the Succession star admitted that he was the 'naughty young actor' who Ruffalo was referring to.
Culkin said he started to have second thoughts about the prank when he smelt that unmistakable whiff heading backstage.
"Uh-oh," he remembered thinking. "I believe it’s happened."
The 42-year-old said he just couldn't resist swapping out a fake joint for a real one when he noticed the bag of the props - and he sat back and 'just watched' when Ruffalo sparked it up.
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"I was 17 and stupid," he said. "I’m like, 'I thought this was a good prank. I’m stupid. Oh my God, I’m so sorry'. But actually, they loved it!"
According to Culkin, after the penny dropped for Ruffalo, the actor told him: "I haven’t smoked pot in 10 years; the second half’s going to be so much fun."
So what we should all take from this is what a good sport - and actor - Ruffalo is. Culkin also explained that the majority of the cast were also impressed with his practical joke too.
He continued: "There was this other actor who had never smoked pot in her life. She goes, ‘Is this what being high is? This is lovely.’
"And then Phyllis Newman comes in and goes, 'I haven’t smoked pot since the 1960s. Thank you, darling'.
"Then the stage manager comes stomping in and goes, ‘I don’t care whose it is, or what happened, but Kieran, give me the joint'.
"I sheepishly handed her the roach and she said, 'Ruin your life on your own time.'"
Culkin promises he has learnt his lesson, saying he now 'knows better' and has no intentions of 'trying to get anyone high on stage' in the future.
The actor said he hasn't smoke weed himself for '20-plus years' - so maybe the whole prop-switch-debacle put him off the plant in the end.
Topics: Drugs, Celebrity, The Graham Norton Show, Viral, US News
Olivia Burke