I have never heard of or seen a pufferfish split open like Sharkky mentioned. I've been around puffers (and coaxed them to puff) for over two decades and never ever seen a problem with them doing it. It is not recommended to allow them to fill with air as this can be difficult for them to expel.
I'm not telling you to do it, but here is the secret to make puffers puff:
Grab a tiny bit of their skin between your thumb and index finger. Gently pinch this skin and tug a little bit at the same time. Works every time.
Precautions: Pay very close attention to what the puffer is doing while you pinch, some of them will make a very determined effort to bite you just before puffing. Be prepared to let go as a puffer with spines begins to inflate - those spines can be needle-sharp and will poke your pincing fingers.
Remember, I'm not telling you to do it, just telling you how to get it done :wink: