You'd have to be stoned to fall for a scam like this, which is exactly what makes it a good scam. An Eventbrite page has been advertising $20 tickets for the annual **free** and un-ticketed 4/20 event in Golden Gate Park according to the Chronicle, sold by an organizer called "Kind Friends Advertisement." With friends like these...
The advertisement, which encouraged potential attendees to "buy your ticket here, to avoid standing in line," has been removed by Eventbrite, and it's not clear how many tickets were sold. "Located on Hippy Hill, in Golden Gate Park, this festival is the only one like it in the United States, and only happens once a year," the ad continued.
Following California's vote to legalize recreational marijuana, this year's 4/20 is the first officially city-permitted incarnation of the event. It's not totally inconceivable that, one day, the event could be ticketed, although "selling out" like that would probably cause an uproar. Still, the event is far from free, at least for the city and San Francisco taxpayers. Last year, 4/20 revelers left 11 tons of garbage in the park, costing $25,000 in cleanup fees. At this year's permitted 4/20, official sponsors will help shoulder that cost.
If you suspect you're being conned, you can always check the Bay Area's Better Business Bureau Page — it's a good way to avoid shelling out for events like last year's NYE party aboard a yacht with K-CI & JoJo that feature no Yacht or K-Ci & JoJo. Save your cash for weed and snacks.