Thought I might get some wisdom here as Google isn't helping much. I was cleaning out my fridge tonight and found a previously-opened glass jar of Trader Joe's marinated red and yellow bell peppers. About half the jar was left, the cap was on tightly, and it had probably been in there for a month or two.
When I opened up the jar, there was a pop and a sound almost like a sizzle, and then some sort of white vapor/gas/smoke came out of the jar. I was kind of freaked out so I just put the lid back on and threw it away.
Aside from there being a genie in my jar of peppers, what the heck could this be? Now I'm afraid to open my jar of olives :)
Edited to add a thought - it is pretty cold in my fridge (I noticed that some parsley towards the back had gotten a little icy) and the jar was way in the back. Maybe it had just frozen a little bit and the white stuff was water vapor?