A glitch caused delays for about 450 Southwest Airlines flights across the country on Sunday, prompting the airline to warn customers to arrive two hours ahead of their flights and to print out boarding passess before arriving at the airports. 

A computer glitch forced Southwest to manually check-in their passengers, some of whom had to have their boarding passess written out by hand. Online and mobile check-ins weren’t affected, but the airport kiosks made life difficult for travelers hoping to check their bags. 

Officials from Southwest say that they have “stabilized” the technical issues as of 10 a.m. Monday, but passengers should expect delays as service returns to normal.