Wayne Whitney writes:

It is conceivable that when the shingles were removed from the
packaging at the time of installation, the cellophane strip, instead
of sticking to the underside of the shingle above in the packaging,
stuck to the tar strip on the shingle below. This scenario requires a
manufacturing defect and a really clueless installer.

Stupidity abounds. The house inspector we used when we bought our house
told us a story about a guy who has a "home handyman" radio show, who
really ought to know something about roofs.

Apparently, somebody installed shingles on a house with the courses
starting at the peak of the roof, not the eaves. It looked good, but
leaked. The show's handyman couldn't figure out what was wrong.

(For people who haven't looked at a roof closely, the problem is that
water flows downhill, and with the roof installed as described, the
water flows off one shingle and *underneath* the one in the next lower
course, thus soaking the roof deck instead of reaching the eaves. On a
properly-installed roof, water flows off one shingle onto the *top* of
the next lower course, repeating this all the way down to the eaves.)

Dave