Watch Machines Go Rogue in the World's Largest Drone Performance

The world’s largest drone display, featuring thousands of drones dotting the sky in the Chinese city of Xi’an, was held during a Labor Day celebration. But occasionally, the drones went rogue.

The show, put on by an affiliate of drone maker Ehang, consisted of 1,374 unmanned aerial vehicles forming messages like “springing Xi’an” and “5.1.” But at time you can see that the drones went off script, resulting in just blobs of light in the sky, instead of clearly recognizable characters. You can see for yourself in the video below, starting at the 0:27 mark.

The drones created the displays as high as 853 feet after taking off from the 600-year-old city wall of Xi’an, according to Quartz. Before the show, Ehang Egret CEO Shi Zheyuan warned Chinanews that signal interference from people’s mobile devices could cause some of the drones to fall out of formation. The city paid Ehang Egret $1.7 million to put on the show. The company had some trouble retrieving its drone afterward as well.