the bag is always source of annoyance for me - the wording should be "might not inflate." Also, critically important on most systems is ensuring the lanyard is pulled so that the unit is activated - it is a terrible shame that some passengers have been found dead in their seats with the masks on in an otherwise survivable incident because the unit was not activated (and sometimes because they took the mask off thinking it was not working because the bag did not inflate.
I think the pre-flight safety instructions do not do enough to emphasize these issues. (of course, many folks aren't even paying attention, but that's a different thread).