Why is it that the worst flair is caused by bright objects just BEYOND the field of view? For example, I've found it's often safer to photograph a bright object slightly in-frame and to crop it out in printing than to crop it out in-camera and risk a the band of flair it may cause.

This problem seems to come up quite often in my shooting and I've always wondered why it happens and how to avoid it.

BTW, for an extraordinary example of this effect, watch the "Jupiter & Beyond" section of Kubrick's "2001" (which was on last night and brought the question to mind -- great flick).