One guess is that you were taking raw or tiff images, your flash card was
slow, and it was taking a long time to transfer the images from the camera
to the flash card.

plonk

I will hazzard a guess that, assuming the roof wasn't too close to
focus on, there wasn't enough contrast in the image to focus properly
on.

I thought I'd post a follow-up for those who responded in a helpful way
about my new Digital Rebel's questionable behavior.

I spoke to one great tech support guy, very young, who suggested the
wildly fluctuating exposure settings in the digitalized portion of the
viewfinder were completely normal.  He pointed me to a really excellent
site, apparently maintained by the Japanese arm of Canon
(www.enjoydslr.com).  I was satisfied that the misfiring camera was my
fault.

However, the next morning, Canon was thoughtful enough to send me a
comprehensive customer service survey--and to ask, at the end, if my
problem had been resolved.  When I noted the techie's professionalism
and courtesy, but stressed that the problem really wasn't resolved,
they gave me a "special" 800 number.  The much older tech support man
at this number put me through a series of tests and concluded the
camera was faulty, and that I should send it back.

His primary test was to put the camera in Program Mode, half click the
shutter, and then see if any of the settings *still* fluctuated.  Did
they ever!  (Hope it goes without saying that I was holding the camera
still and pointing it in the same direction, with no change in ambient,
indoor light.)

Anyway, he was very confident that wildly fluctuating exposure readings
meant something inauspicious.  I sent the camera back to Amazon, just
got my new one, and the fluctuations have not reappeared in this unit.

Just wanted to follow-up with you guys.  Everyone who gave
passive-aggression-free help, wish I could buy you a round.