One Michigan State University doctor had several patients tell him that they'd passed kidney stones after riding roller coasters. Specifically, one of the rides at Walt Disney World.
MSUtoday.msu.edu has the full story here on the particulars of the anecdotes that led to the study that showed a 70 percent success rate in passing kidney stones from riding an amusement park roller coaster, but keep in mind that where you sit does make a difference! Get all the details here.