Resolving airwires in EAGLE for valid wires
It looks to me like the connection points of the track and the rectangular pad are misaligned - perhaps there is a short 45 degree segment of track placing its "hot spot" offset from the pad's hot spot. In Protel, I'd set the tracks and pads to show as outlines to more easily see, and correct, the fault. The outline tracks mode can show up other confusing things, like multiple vertices along a straight track, which can make dragging a track section difficult.

@Tom Carpenter reports that in Eagle if you want to see the outlines, you just change the fill mode in the layers panel.

A similar fault that I've experienced in KiCad occurs when you place a track along a row of pads - KiCad (and perhaps other programs) won't recognize a track simply passing over a pad as connected unless you actually click on the pad as you pass it.