"The Annual International Wizard Gardening Competition, for example, caused many problems. So called 'crop circles', really entries in the Contorting Cereals class, caused sensation among Muggles worldwide"
-- from the report "A Study into Muggle Suspicions about Magic" (DP1)
Gardening competition for wizards (DP1)
This includes an event entry for Contorting Cereals, which has caused confusion for Muggles who see the results, calling them 'crop circles'. A report from a Ministry of Magic sponsored committee recommended that the International Confederation of Wizards ban this particular class from the annual competition (DP1)
While the date printed on DP1 is 31 July 1998, the timeframe for these events is 1992-1993.