Both bulging and loss of seal are signs of spoilage. If either of these occur, discard the food. It could be dangerous. Spoilage organisms must be growing in the food and producing gas. The build up of pressure inside the jar results in bulging or loss of seal. Growth of spoilage organisms can be attributed to under processing (product was processed by wrong method or too short a time) or to a hairline crack in the jar which permitted entry of organisms during storage. Never taste home canned food when the lid has bulged or come unsealed.