More than 600 people paid $50 (£29) for a ticket to the event, which promised wizard-themed drinks, food, games, live music and even debauchery and mischief.
After queueing for up to an hour in freezing temperatures, the guests were directed down to the basement by a whiteboard with the scrawled message: “Magic this way”.
It went even more quickly downhill from there – as they discovered the “Wand Making Station” was a table scattered with chopsticks and glitter.
The advertised “Wizard Sweater Cookies, Cauldron Cakes and Butterscotch Beer Popcorn” turned out to be a selection of supermarket snacks and finger food.
A "magical photo opportunity" was in reality a recreation of Platform 9 3/4 at Kings Cross Station, involving a fake brick wall and a free-wheeling trolley.
And instead of the promised “Magical Blood Shots and Hair of the Multi-Headed Dog”, partygoers were offered only normal “muggle” cocktails like rum and coke, and gin and tonic.
“It felt like an insult to every Harry Potter fan,” one guest complained. “Half the people just left”.
Youtube user NerdEJackie, who published a video of the festival with the title “I got scammed at a Harry Potter party”, urged fellow guests to request a refund.
“The whole vibe was a huge downer,” she said. “So many people are dressed up amazingly but no one is happy, everyone has been complaining non-stop.
“It was supposed to be adults only but it seems to have been organised by a 10-year-old. It’s a magical disaster.”
The underwhelming experience even prompted comparisons with the notorious Fyre music festival which left guests stranded on an island in the Bahamas in 2017. “It felt like we had our own little Montreal Fyre,” Georgina Elias told CBS.
Another guest, Georgiana Sketchyova, set up a Facebook group for anyone trying to get refunds from the organisers, LOL Event Group. Several people reported that their requests for refunds had been refused.
Others claimed the company had changed their description of the party – which urged fans to “join our wizard-inspired adults only festival as Montreal turns into a world of magical wonder” – to remove all the Harry Potter-themed references.
A caterer also complained that the organisers had left him $2,500 out of pocket by cancelling hours before the event started.
LOL Events responded to one disappointed guest by asking them to “give us another shot”, adding: “We are a new company trying to bring magical joy to multiple cities around the country, I am sorry that the event did not live up to your anticipations.”