At all cost? That is a hard one. Depends a bit on the genre but here is a short list of three that I think should be avoided.Killing of a main character only to resurrect him/her to make a happy ending.
To kill a main character is a good way to make a climax, a good writer knows that. A bad writer decides to resurrect said character, without any consequences. Compare with zombies or vampires, creatures returned from the dead and suffers the consequences. It is a lazy way to give the readers what they expect/want but not in a satisfying way.A stupid and not evil villain.
A combination of two clichés:
1. A villain that is stupid. Examples, a hundred of them: http://www.eviloverlord.com/lists/overlord.html
2. A villain that is not evil. My example is from Harry Potter. Voldemort was evil, they spend all the books telling the reader about how evil he was in the past - what he does in the actual story? Nothing really. Darth Vader from the Star Wars Episode 4,5 and 6. He kills more imperial officers than he does rebels - he just looks evil.Making the most unimportant character the most important character.
Remember that man/woman that the author mentions for on or two sentences? He/she will save the day or give the protagonist just what he needs to do it.