The first time we meet special agent Fox "Spooky" Mulder, his office includes a striking piece of decor: a poster depicting an old-school, flying saucer-type UFO hovering above a treeline, with the words I WANT TO BELIEVE emblazoned in all caps below. The artwork was an instant must-have for The X-Files fans (you can still order one on Amazon, if you're in the market), but it's also the series' original, longest-lasting, and most ubiquitous easter egg.
The poster first appeared in the pilot and was seen on-screen in every subsequent season along with both X-Files movies (it also provided a title in the case of 2008's The X-Files: I Want to Believe) — and it made two key appearances in the 2016 miniseries that brought Mulder and Scully back to the small screen after a decade-long hiatus.
By then, Scully has her own copy ("What are you doing to my poster?" she says, after catching Mulder using it as a dartboard); but the more memorable of its scenes came when Mulder wandered through the wreckage of his former office at the very start of the show, and callously shredded the I WANT TO BELIEVE poster with his giant skeptical feet, a moment which absolutely scandalized the show's fans. (David Duchovny, however, was untroubled by the sacrilege he'd just committed; at Comic-Con, when asked what it was like to shred the poster, he replied, "I felt nothing.")