A co-pilot who spilled coffee on the instrument panel of the Serbian president's plane and then hit an emergency button while trying to mop it up, caused the jet to plunge, an investigation showed.
President Tomislav Nikolic was forced to abort an official visit to the Vatican on Friday after one of the engines of the plane carrying him to Rome shut down.
A report by Serbia's civil aviation authority blamed the incident on the co-pilot's brush with a coffee cup.
"Co-pilot Bojan Zoric accidentally knocked over coffee onto the instrument panel and, in trying to mop up the coffee, activated the 'emergency slot extension' which caused the plane to lose altitude," the report said.
One of the Falcon jet's three engines briefly shut down as a result, the aviation body said.
While the pilot and co-pilot managed to "rapidly retake control of the plane" they were forced to turn back to Belgrade, causing the president to miss his appointment with Pope Francis, the report said.
Belgrade had billed the visit as a chance to consolidate ties between the Vatican and Serbia, a majority Orthodox Christian country.
Mr Nikolic attended the pope's inauguration in 2013.