This article is from the Food Preserving FAQ, by Eric Decker email@example.com with numerous contributions by others.
a. Off-standard jars and/or lids.
b. Chipped or uneven sealing edge.
c. Using one-piece caps instead of two-piece lids.
d. Screwbands are rusty or bent providing poor contact.
e. Bands not screwed down tightly enough before processing.
f. Sealing edge not clean. Wipe edge well before placing lid on rim.
g. Liquid siphons out of jar during processing taking food particles on
to the sealing edge.
h. Insufficient heat during processing. Air is not evacuated from jar, so
a vacuum seal never forms. [In pressure canning the EXHAUAST phase is
i. Lids were improperly prepared before placing them on rims. Most lid
manufacturers require some pretreatment (heating, boiling, etc.). Use
lid strictly according to the manufacturers specifications. There are
significant differences between the lids of various manufacturers. -ED
j. Rapid, forced cooling of a pressure canner can cause a rapid pressure
and temperature change inside the canner causing the liquid to "boil" out
of the jars, leaving particles on the sealing rim and unsealing the jars.
Canners should not be "forced" into cooling rapidly by submerging [ no
dousing or spraying either - ED] them in water or by adding ice.
k. Insufficient processing of raw-packed food; the air may not have
been completely driven out of the food leaving residual air in the jar
so the seal does not form. l. Use of canning procedures which are not
recommended such as open kettle canning, microwave canning, and oven
canning. Use USDA recommended procedures.