It’s an unnerving bang.
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Holy Heck!! I’ve always told folks that could happen, but I’ve never seen it actually happen before.
Most vacuum investor bell jars come with a label saying to only push down on the rim, not the top.
Now we know why! Hope no one got hurt!!
Jeff
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Luckily the vacuum pulls everything “in”. But yeah, might need new underwear.
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This happened to me a few months ago. Thing is I knew about this I just was tired that day I guess. Part of me thought “I think it needs a little better seal” (not realizing it was already pulling full vacuum) and pushed down on the top a bit. BOOM! Shards of acrylic everywhere. My jar looked exactly like yours. Luckily I had no cuts and I was just testing it so no flask was lost.
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I’m glad I’m not the only curious one here lol 
it certainly was a surprising moment!
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Yes, definitely don’t ever press down on the top of a bell jar when vacuuming. It’s safe to press on the bottom edge, but not the top. Glad no one got hurt!!
Jeff