The typical situation where this happens in a chem lab (where I learned about this stuff) is having glass tubing immersed in a dewar as a vapor trap for a vacuu

The typical situation where this happens in a chem lab (where I learned about this stuff) is having glass tubing immersed in a dewar as a vapor trap for a vacuum pump. In normal operation, the tubing has near-vacuum inside, and the liquid nitrogen condenses any volatiles that might get into the vacuum system. But if you open the vacuum system to the air and leave it immersed in the dewar, then you can condense a considerable amount of oxygen in the bottom of the vapor trap... which is already covered in whatever organic gunk you were trapping in the first place. I post this not because I think people keeping a thermos of liquid N2 in their garage need to worry about it, but because it's such a nonobvious way to blow yourself up with what is in all other respects a docile substance.