Good Idea/Bad Idea in RPGs:

Good idea:
Asking the thief to check out the passage ahead of you for traps.

Bad idea:
Insulting the thief and then asking him if the passage ahead of you is trapped.
--------------------
Good idea:
Casting fireball to destroy the waves of goblins rushing you.

Bad idea:
Casting fireball at the waves of onrushing goblins in a 20x20 room.
--------------------
Good idea:
Lining up your fighters shoulder to shoulder to cut down the line of kobolds

Bad idea:
Forgetting to make sure there's not an invisible mage with a lightning bolt off to your right just waiting for the fighters to line up shoulder-to-shoulder.
--------------------
Good idea:
Dimension dooring next to a line of pole-arm bearing goblins to hit them with a cone of cold.

Bad idea:
Casting the cone so that it misses the one right next to you.
--------------------
Good idea:
Pulling the pin and throwing the grenade at your enemies.

Bad idea:
Forgetting to make sure that the bullet-proof window between you and the enemy is open before throwing the grenade.
--------------------
Good Idea
Charging round a corner with an elephant gun after a mad occultist.

Bad Idea
Charging round a corner with an unloaded elephant gun after a mad occultist.
--------------------
Good Idea
Climbing a tree to avoid a Monster.

Bad Idea
Climbing a tree in Plate Armor to avoid a Monster and falling on your back.
--------------------