This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
a. Processing at an incorrect temperature. Can occur with:
1. Inaccurate pressure canner gauge.
2.Failure to exhaust canner.
3. Failure to make altitude adjustment.
4. Heat source fluctuates--inaccurate pressure or fluctuating pressure.
5. Water not at a rolling boil when jars are put into canner.
6. Water not covering jar caps by 2" throughout processing.
7. Water not at full boil throughout processing.
8. Insufficient processing time.
9. Use of canning procedures which are not recommnended. Recommended
procedures (USDA) are based on the time it takes to achieve a temperature
which will sterilize the food in the jar.
b. Improper cooling of jars after processing.
1. Failure to remove jars from canner when processing time is up (or
when pressure gauge reads 0).
2. Failure to set jars at least 1" apart during cooling.
3. Covering jars which retains heat. Vacuum does not develop.
4. Attempting to cool either the canner or the jars very rapidly.
c. Using damaged (freeze damaged), spoiled, under ripe or over ripe food.
The pH may not be correct for the type of processing you used (water bath
d. Very large number of microorganisms due to spoilage, bruising, etc. A
very large number of microorganisms present on the food which are not
destroyed in the usually recommended amount of processing time.