Cryogenic storage vials are designed for VAPOR PHASE STORAGE in liquid nitrogen freezers. This means that they are designed to sit in the cloud of extremely cold nitrogen gas that sits just above a small reservoir of liquid nitrogen in the bottom of the freezer. Leakage of liquid nitrogen into the vial occurs with the freezer is overfilled and the vials are immersed in liquid nitrogen. This problem can be avoided by not overfilling the freezers with liquid nitrogen.
Visually check each cryovial prior to filling to ensure there are no defects around the rim. Cryovials should never be re-used.
When removing samples, pause for a moment in the neck of the dewar before bringing them into the room atmosphere - if one is going to pop, it will usually do so early in the warm-up process.
The importance of gloves and face shield can not be overemphasized. Tubes stored in liquid phase dewars, where the ampules are in canes is especially hazardous. Since nitrogen freezers tend to be located separate from the labs, full face shields and gloves should be available near the nitrogen freezers so no one is tempted to pull a vial without protection because they forgot to bring a shield with them.