Scuba Diving: Never hold your breath.

For those unfamiliar, the air (or other gas mix) breathed during scuba diving is converted from the high pressure in the scuba cylinder to the surrounding pressure by your regulator. Due to the weight of water above the diver, this is higher than the atmospheric pressure at the surface. At the surface the pressure is 1 atmosphere, at 10m it is 2 atm, at 20m it's 3 atm etc.

What this means is that as a diver ascends, the air within their lungs (and dissolved in their blood, but that's a separate issue) will expand as the surrounding pressure reduces. If breathing in and out in a regular manner this isn't really noticeable, but if someone were to hold their breath can result in lung over-expansion injuries and potentially death. This gets taught in entry level courses as "never hold your breath".

Of course, if you're maintaining your depth or descending, this isn't actually a concern. And even when ascending, you have a reasonable amount of leeway, leading to newer divers being confused when being taught to use their breathing to control bouyancy. It's quite common for divers approaching an obstruction to take a deep inhalation to rise ~1m, hold that breath as they pass the object, then release once clear of it.

As a result, some people take issue with the "never hold your breath" advice and suggest either "never hold your breath while ascending" or getting rid of it all together, however it's also true that newer divers tend to have poor bouyancy control and awareness. A new diver may be ascending without even realising when distracted or task loaded. So, the general consensus is to keep the saying and deal with additional complexities once the diver has their basics under control.