Newbie Mistakes
This list is a bunch of so-called "newbie mistakes" because they're basic and simple rules of the game fighting players. Anybody with experience has died because of the reasons here, and probably will again, because they forgot. These mistakes are all avoidable - with exception of harassment from thieves (not exactly common) making them is your fault.

Not having a recall potion

A recall potion takes you to the temple of your hometown, regardless of where you are. Only a few places, and some spells, can prevent you from using recall potions.

No purple potions

Purple potions mean having sanctuary. With sanctuary, damage taken is greatly reduced. If you are planning to stay in a fight, use this, especially when you know the enemy has better equipment than you. There are some other useful preps to find.

Gyvels in sack

Sounds ridiculous possibly but it really does happen. If you're blinded and the gyvels are in the sack, you won't be able to use them. Keep out four or five, they don't all have to be out. A variation of gyvels in sack is to have bought weak ones, its not that expensive to buy level 50 gyvels.

Using the wrong weapon, with the right one to use in inventory

Use this checklist:
  • You're wielding the right weapon (e.g. water cube vs dwarf)
  • Important potions are in your inventory
  • All the unimportant potions are in your sack
  • Other important weapons are in your inventory (e.g. polearm for overheads)
  • All the unimportant weapons are in your sack

    The only possible advantage to having stuffed all your good items in sack, is that when they're looting you, you often get the sack back, so you get back all the good items. You might win if you actually use them.

    Spamming

    If you want to win fights, you're going to need to use skills you've practiced. But this doesn't mean spam and hope for the best. Whenever you enter skills, wait most of the command's lag delay out before entering the next. This will leave you able to change your mind. (A classic example is spamming disarm/overhead/other bad choice skill until you die.)

    Area of effect at the temple healer / shop/ etc

    Fireballs, blindness dusts, etc. These are good attacks but not wise to use them upon level 60 mobs.

    Fighting hungry

    You would be surprised how many people don't bother to feed character (eat & drink) mid-battle. That's damage, and bad regen. It might even kill you at low health. Jug/Beef are spare items for sack.

    Playing overly conservative

    While its good to retreat to fight later, it doesn't help to just run away at the first sign of potential danger. This isn't making the most of all your health points, gold spent into scrolls, etc.

    Refusing to run

    Sometimes a battle is just a lost cause against overwhelming odds. When this happens, the best thing to do is back off and wait for support. It might also annoy your enemies.

    Staying in the dark

    You can use the WHO command to see if anybody in your pk range is about. The WHERE command shows the people in your area. If you never bother to use these, especially while levelling, you might get attacked when you least expect it. This isn't always a newbie mistake, though, its also a mistake made by tired levelling groups.