The creation of complex active nanosystems integrating cytoskeletal filaments propelled by surface-adhered motor proteins often relies on the filaments' ability to glide over up to meter-long distances. While theoretical considerations support this ability, we show that microtubule detachment (either spontaneous or triggered by a microtubule crossing event) is a non-negligible phenomenon that has been overlooked until now. The average gliding distance before spontaneous detachment was measured to be 30 +/- 10 mm for a functional kinesin-1 density of 500 mu m(-2) and 9 +/- 4 mm for a functional kinesin-1 density of 100 mu m(-2) at 1 mM ATP. Even microtubules longer than 3 mu m detached, suggesting that spontaneous detachment is not caused by the stochastic absence of motors or their stochastic release due to a limited run length.