For an oxygen-rich atmosphere, it's important to always make sure that gases expelled from the confined space through ventilation do not come into contact with internal combustion engines, flames or other sources of ignition that may cause an explosion or accelerate burning.
While this might seem obvious, it is sometimes overlooked in the rush to clear a confined space and make it safe for entry.
When sizing up a potential confined space for entry, also be aware of where you are placing the atmospheric contents of that space, and think carefully about how they may affect the area they are ventilated toward.