Mostly going slightly deeper to mine after I mine out all iron in easy reach, then getting ninjaed by skeletons. Oh, and falling creepers.
Other times, I just starve because I couldn't find enough iron to mine before I ran out of food.
And I sometimes get overexcited and forget to go home at night, or go just slightly too deep.
Edit: Also I sometimes take a long break, then forget a good 1/4th of the survival tips and tricks until I mess up.
Or simply forget to look at tool durability.
Falling creeper? Don't walk anywhere that could have a jumping spot from above.
Keep building material like dirt to block up areas in caves that could bring in unwanted guests.
Be smarter than the skeleton, use line of sight to bring it in close, use blocks to stop it from shooting you.
If you get low on food, go on hunting trips, you don't need to return home at the end of the day, you just need to create a safe area that you can work in during the night and exit safely.
I find long hunting trips to be especially beneficial for leaving markers to find your way back home to help with future deaths.
Honestly it's just playing it that will help with survival. Each death you can reflect as to what killed you and change your play-style to compensate