we do the annual boat course for local beach lifeguards.
Some years ago on a nice hot summers day (remember those?) James, a lifeguard went into the water to do a test inflation of a manual lifejacket-something their manager insisted on every year. Before entering the water he opened up the velcro fastening around the lifejacket for some reason or other ("to help it"!?!). In the water he pulled the inflation cord and NOTHING happened!
Amidst a chorus of abuse from his mates, including "if at first you don't suceed then skydiving is not for you!" he climbed back into the boat where we found that when the cord was pulled it was simply rocking the bottle and it was only the velcroed up outer shell that kept the thing rigid enough to fire it.
Bad design-lesson learned!