This article is from the Food Preserving FAQ, by Eric Decker email@example.com with numerous contributions by others.
a. Insufficient processing time.
b. Processing temperature too low (water not at a full boil at beginning
of processing or drops below full boil during processing).
c. Water not 2" over jar lids.
d. Packing foods raw that should be precooked (pears).
e. Liquid loss during processing causing fruit at the top to be out of the
f. Lack of appropriate pretreatment for light-colored foods.