My mom had one of those relief valves open up. It was one of those rubber or lead plugs that just open up a small hole in the lid, then vents the pressure. Designed to prevent over-pressure explosions, this one just got old and weak.
We had pressure cooked chicken broth an bits of chicken raining down from the ceiling. My Mom just sat down and cried as the chicken broth spewed upwards. Nothing she could do about it, supper had to be postponed and she had a sticky mess to clean up. The entire kitchen had to be scrubbed, ceiling to floor, inclusive, including the stove and counter tops.