Here's a list according to me that are the most common mistakes whilst offroading:
Going downhill on brakes
No winch blanket
Not listening to spotters
No idea where the steering is pointed
Not carrying recovery gear
Not carrying medications / food / water
Entering water with a splash
Not walking an obstacle
Having no clue where the diff is located
Not using gloves for wire ropes
Not coming down in reverse gear after a failed ascent attempt
No roll bar / seat belts especially for open vehicles
Not carrying adequate tools
Carrying too many people in the car during obstacles leading to extra loads and a twisted prop shaft
Entering deep water without snorkel
Poorly fitted snorkel
No breather tubes for GB / Diff
Entering water with an extremely hot engine
Not understanding tip over angles
Overspeeding on obstacles
Thumb in driving
Not doing a pre offroad check on vehicle before getting the vehicle on a OTR trip
No spare tyre
Offroading alone without a back up vehicle or telling someone where you are going
Not knowing when to get off the gas and leading to burning of the clutch
Driving on tarmac with 4x4 engaged
Forgetting to engage the hub locks
Wrong gearing
Wrong tyre pressure
Not securing loose objects in the vehicle
Not checking water depth before entering
Spectators in kill zone during recovery
Wearing improper footwear while driving
Entering a descent sideaways
Trying to cross a river without judging the flow rate
If you've not offroaded before, don't attempt it alone
Don't keep your hands outside the vehicle
Not knowing basic DIY
Venturing in 4WD terrain with a 2WD vehicle