The "Crucible curse" (also known as "The curse of the Crucible") refers to the fact that no first-time snooker world champion has retained his title the following year since the tournament moved to the Crucible Theatre, Sheffield in 1977.
The last player to successfully defend his first world title was John Pulman in 1964, when the championship was still decided by challenge matches, and the tournament had not yet moved to the Crucible Theatre. Since 1977, 17 world champions have experienced the curse, the most recent being Stuart Bingham at the 2016 World Snooker Championship.
Crucible curse Wikipedia
The following first-time champions did not retain their title the following year. Of the seventeen players to have won their first championship at the Crucible Theatre, six lost in their first match as defending champion (Terry Griffiths in 1980, Steve Davis in 1982, Dennis Taylor in 1986, Graeme Dott in 2007, Neil Robertson in 2011 and Stuart Bingham in 2016) and only two went on to reach the final: Joe Johnson in 1987 and Ken Doherty in 1998.
In years not listed above, the title was won by players who had already been world champions, and thus the "curse" did not apply. Three players have won consecutive titles at the Crucible: Steve Davis (1983–1984 and 1987–1989), Stephen Hendry (1992–1996) and Ronnie O'Sullivan (2012–2013), but only after they experienced the "curse".
Even before the World Championship was staged at the Crucible, no player in the modern game successfully defended his first world title. Of all first-time world champions, only three retained their title at the next World Championship: Joe Davis in 1928, his brother Fred Davis in 1949 and John Pulman at his first challenge match in 1964.
Additionally, Ray Reardon, Alex Higgins and John Spencer won the World Championship before the event was moved to the Crucible in 1977 (and failed to retain it the year after their first title); they all also failed to retain the title after winning it for the first time at the Crucible.