Bleach stone fell into a hole and the hole was built over thus locking the gas where it cannot move or change pressure. This hole contained Carbon Dioxide and although it was not 2000 grams of pressure it should not have allowed the bleach stone to off gas as the Chlorine has no where to go. This was just deleting mass in the form of chlorine gas. Here are a few pictures below and if Gyazo works here is a video. Hope this helps or if this is intentional please let me know.
Video of the incident:
Here are a couple of pictures.
Have a gas in a hole, Carbon Dioxide in this case, and build tiles that displace bleach stone into said hole. Once this hole is covered over by another tile it should stop off gassing, but it just continues without the gas going anywhere. The gas doesn't appear above the tile or anywhere around it.