There's little more ironic than a flask saving your life. However, it's undoubtedly become a cliche for films (westerns, in particular). This is what a flask looks like post bullet deflection... It's your very own luck flask.

How many times have you seen the hero (or villian) saved by the fact that the bullet hit the flask, deflecting it from their heart...? Oh, about 20 times or so.
And if you haven't, get out there and rent a spaghetti western!

This lucky flask has an indent emulating where the bullet saved your life. Or your butt, depending upon where you keep your flask.