1. Raising an army of Undead to conquer Tamriel is never as good an idea as you thought it was to begin with. For one thing undead are really, really stupid. And if they're not then they're litches. Do you really think a litch is going to let feeble little you take over the world instead of him? Besides, it didn't work for Cameran the Usurper or Potema the Wolf Queen. Why do you think it'll work for you?
2. Do not stand in pools of oil. I can't stress this enough.
3. It's been said often, but no one seems to remember that if the person next to you screams, bursts into flame and goes flying through the air it is not "the wind", "too much ale", or "nothing after all."
4. And if you should notice that your old buddy is dead, for Sithis sake don't just stand over the body going "Whaaaa--"
5. Pressure plate traps should be indistinguishable from the surrounding floor. Not decorated with cunning little patterns, or metal plates, or anything. Try tripwires instead. They're nearly invisible.
6. Making a door with a revolving plate lock with rotating symbols with 27 different combinations is a good idea but if the person opening it gets the wrong combination something should happen -- poison darts, flame throwers, a naked dancing wood-elf, or something else really painful. Also, putting the combination to the plate puzzle on the key is a really really stupid idea.
7. If you have spent the last two centuries researching an obscure artifact and have discovered it is hidden in a ruin in the wilderness, you should probably not leave your journal revealing this information laying around on your bedroll. On the other hand, you're probably dead already, so it doesn't matter.
8. Do you really think all adventurers are dumb enough to grab the cheap iron gauntlets off the pressure plate that sets off a dart trap? Well, ok, most of them are, but eventually you're going to run out of minions willing to put the darn thing back.
9.Calling someone in full Daedric plate, wearing a black glowing sword on her hip a "milk-drinker" will not contribute to your leading a long and healthy life.
Almost forgot:
10. Running to battle the foe while the person behind you is planning to throw a fireball -- bad idea.