Hint:
Don't wear one of these critters when stepping down into a stream to cross it, unless you are first assured that the lid is latched.
The act of bending over has this nasty habit of placing your shoulder near-level with your waist, with a forward motion that may (just may) allow the contents of those nice, slippery tubes, to carry forward... into the drink.
Let's just say that Saylor's Creek ate more than a few of my blanks... Accidentally. And that water is still coooold in April.