A long, long time ago [before 1970], IBM was about to introduce a
new disk drive that had higher storage density than any it had
built before.  This disk drive came in multiples of eight,
stacked two high, four wide, and was immediately dubbed the
"pizza oven" by its users.

Anyway, the new drive was introduced with much fanfare.  A few
weeks later, its first customer had their first head crash.  Then
their second, and soon after that, their third.  The chagrinned
IBM service people bundled up the entire drive and took it back
to their lab in Poughkeepsie, or Kingston, or wherever.

There they replaced all the heads, put in brand new packs, and
let it run.  It ran for more than a month with no problems, so
they carted it back to the customer site.  They left the heads
alone, put the customer's packs back in, and let it run.

A few weeks later, the heads started crashing again.  They repeated
the whole process, but again the heads would not crash in the lab.

Finally, someone got the bright idea of examining the crashed
heads under a microscope.  They found them clogged with some
hard resinous substance, which they subjected to chemical
analysis.

The substance turned out to be the glue from the quality control stickers.
These stickers were affixed during final inspection to each disk pack
sent to customers, but not to the test packs they used in the lab.
After a few weeks, the glue dried out and started flaking off;
some of the flakes eventually found their way into the heads and
caused the crashes.