“Slipped band” is actually a misnomer. It suggests that the band “slips” over part of the stomach, which is actually incorrect. In most cases, the band itself is scarred down to the surrounding tissue, as the stomach above and below the band is what migrates. This results in the stomach being partially trapped above or below the band, which causes nausea, vomiting, and abdominal pain. If it is not urgently treated, it may cause erosion and perforation of the trapped stomach tissue.
The treatment may be as simple as completely removing the fluid in the band, which may allow the stomach to return to its proper place. Though in my experience, the majority of the cases require a surgical intervention and removal of the band.