When I go to dispense liquid nitrogen after someone else has just used it (say in the last 30 minutes or so), the copper pipe used to transfer the liquid is covered in frost/ice.
This makes sense, as the pipe gets incredibly cold, condenses water out of the air which freezes extremely quickly due to the vast difference in temperature (77K for liquid nitrogen vs. water freezing at around 273K).
What I find confusing is that once I start dispensing the nitrogen, this frost/ice appears to 'melt' into a liquid, this seems less intuitive to me. My first thought is that so much new water is condensing out of the air that the frost/ice gets 'wet', however I'd expect this to freeze very quickly and the ice to eventually get so thick that this issue of wetting became irrelevant. Alternatively, one could imagine condensing something else out of the air which is indeed a liquid at these temperatures such as oxygen, but in this case I don't observe the characteristic blue colour.