1) Not picking up a mirror in Portal of Evil.
I lost the count of the number of times you have the opportunity to pick up a mirror or mirror like object in Portal of Evil. When you finally meet Horfack, you realise why it is so important. You also realise that Peter Darvill-Evans is being very helpful. What would you rather face - a mindless skill 8 stamina 10 zombie or a powerful skill 10 stamina 20 fighter who reduces you skill by 2?
How blatant does Dave Morris have to be when he says 'Guard it (the map) with your life as you cannot complete your quest without it.' If you don't have it when you reach the desert then your bleached bones deserve to turn to dust in the wasteland.
3) Throwing Singing Death into the pit in Sword of the Samurai.
You've come all this way for the sword, it boosts your initial stats and you still throw it away and destroy it. Even the demon Ikiru isn't that evil.
You have gills. You can see a big underwater city. If you swim to the surface you will be seen by the ship load of evil pirates who have just thrown you overboard to drown. How blatant does the clue need to be? Explore Atlantis.
5) Putting on a slave collar in Chasms of Malice
There are plenty of random deaths in Chasms of Malice but putting on a collar that only the slaves of your enemy wear is not random and although you probably will die in about four paragraphs time, you shouldn't die in this way.