A flight was delayed for more than two hours on Thursday after an Italian Ivy league professor’s math equation was mistaken by a passenger as a terrorist threat.

On Thursday evening, Guido Menzio, a successful economics professor at the University of Pennsylvania, boarded a flight from Philadelphia to Syracuse. As the 40-year-old professor was waiting for the flight to take off, he began working on a differential equation related to a speech he was slated to give at Queen’s University in Ontario, Canada. Soon after Menzio got to work, his seatmate, who said she was feeling sick, slipped a note to the flight attendant. Surprisingly, the plane turned around and headed back to the gate. Menzio was then escorted off the plane and questioned by an official, who informed him that he was suspected of terrorism.

“I thought they were trying to get clues about her illness,” Menzio told The Associated Press. “Instead, they tell me that the woman was concerned that I was a terrorist because I was writing strange things on a pad of paper.”

The situation was ultimately resolved after Menzio explained he was working on an equation and showed the staff his calulcations. He returned to his seat and, after being delayed for more than two hours, the plane finally took off.

Thursday’s event was just the latest in a string of related incidents of racial and religious profiling on airlines. As anti-Islam rhetoric escalates in the United States in the wake of last year’s terrorist attacks in Paris and San Bernadino, an increasing number of Muslims, Sikhs, and people of Middle Eastern descent are reporting incidents of racial profiling as they attempt to board planes.

In April, Khairuldeen Makhzoomi, a 26-year-old Iraqi refugee and UC Berkeley student, was escorted off of a plane after he was overheard speaking Arabic on the phone. A passenger reported Makhzoomi to Southwest staff after hearing him say “inshallah” — a common Arabic saying which roughly means “God willing” — to his uncle. Just two months earlier, a well-known Sikh actor was barred from boarding an Aero Mexico flight to New York City after he refused to remove his turban.

As ThinkProgress previously reported, four passengers of Middle Eastern descent in November were taken off of a Spirit Airlines flight for “suspicious activity,” which actually consisted of one of the passengers viewing a news report on his phone. The same month, two Palestinian-Americans were barred from boarding a flight in Philadelphia after a passenger complained that the pair’s Arabic conversation made her uncomfortable. And in December, a Sikh attorney and filmmaker on a Delta Airlines flight was asked to show her breast pump to passengers to on the aircraft to “prove” she wasn’t a terrorist.