Upper-case and lower-case letters used according to taste, punctuation marks used as you feel it, and not according to the text’s logic, foreign names spelt according to the author’s fantasy. What is the enemy of proofreading? A messy original text. The more errors in the source text the higher the risk of mistakes appearing in the correction process and leaving those hidden in the thicket of proofreading symbols. For this reason it is key to use legible proofreader’s marks.
Each language has its dictionary standards, but publishers also have their preferences that are often difficult to discuss with. We say: “Tough luck! Just don’t go too far in those compromises.” We gulp all those names, weekdays or months spelt with a lower-case letter, even though they should be always spelt with a capital letter. But many companies have their own ways…
We can see a plague of “trendy” words, which are often misunderstood and used much too often, becoming buzzwords that no longer carry a meaning.

Authors seem to particularly like pleonasms (i.e. burning fire, the month of May) and tautologies, and they absolutely love words such as possess, realise or create (when they mean have, carry out or make). We are a truly rich society of people who possess mobile phones, credit cards, cars etc. But in fact, we can only possess a fortune, land or skills. That’s all.
People often use words without really understanding their meaning, e.g. ‘ironic’ for ‘amusing’, or use plural where the singular form should be used (‘bacteria’ is plural for ‘bacterium’, ‘phenomena’ is plural for ‘phenomenon’).
Idioms are an important element of any language. They are like butterflies, adding lightness, colour and emotion to the language. But you need to know them for sure, otherwise you’ll get a mess. ‘I could care less’ does not mean what you actually mean, and don’t say ‘literally’ if you don’t mean it for real.

Another nightmare for authors (and proofreaders) is punctuation. No one seems to care about the correct use of the apostrophe or the punctuation mark. To show you the importance of punctuation, let me quote a joke about bad punctuation that Lynne Truss used as an inspiration for the title of her book on punctuation:

A panda walks into a café. He orders a sandwich, eats it, then draws a gun and proceeds to fire it at the other patrons.
“Why?” asks the confused, surviving waiter amidst the carnage, as the panda makes towards the exit. The panda produces a badly punctuated wildlife manual and tosses it over his shoulder.
“Well, I’m a panda,” he says. “Look it up.”
The waiter turns to the relevant entry in the manual and, sure enough, finds an explanation. “Panda. Large black-and-white bear-like mammal, native to China. Eats, shoots and leaves.”