Had an identical experience at IAD a few years ago, except I was Elite and in the F checkin line (with a paper ticket no OLCI).

Reached the front of the v slow F line at exactly 2pm for a 2.45pm flight - how do I know it was exactly 2pm? Because the sole F agent turned his back on me, muttered "it's 2pm and I'm going off shift" and left the F checkin unmanned. We all drifted off to other lines and by the time I made the front again it was too late.

Made my displeasure very clear.

The security-guard detailed to escort me away fully agreed that I had been justified in causing a disturbance.