Delta Air Lines is spending $150 million this year on technology upgrades, including a better mobile app. But what CEO Ed Bastian never saw coming was a vulnerability to its reservations and operations servers that would cripple his airline for days.

A computer outage Monday morning grounded planes around the world, stranding thousands of summertime travelers left in limbo as Delta struggled to sort out the mess, its reputation as an on-time airline severely tarnished.

Problems lingered through Wednesday, and the Atlanta-based airline has canceled more than 2,100 flights so far this week. Thousands of other flights were delayed.