Another of the errors that I often see is inappropriate word choice following from an apparent failure to understand the difference between "in" and "into", and between "on" and "onto".
This difference is actually quite simple, and can be summed up by saying that where there is a "to", there is always movement. It's quite logical, isn't it - when you move, you go from one place TO another place. Hence, whenever you are writing about some situation where there is movement, whether implied or express, then "into" and "onto" are the go. When the situation is static, it's "in" and "on".
For example, consider the old and popular saying, "The cat sat on the mat." A cat sitting is not going anywhere. Therefore "on" is quite appropriate. "The cat sat onto the mat" is just wrong, right?
However, this isn't where the majority of people go astray. The common mistake is in using "in" or "on" when the situation is dynamic.
Consider this sentence: "He placed his gun on the desk."
This is another of the same form as "The cat sat on the mat." When you place something somewhere, there it sits, static and unmoving. "On" is therefore quite appropriate - but look at what happens when we change the verb to something that indicates movement.
"He slid his gun on the desk."
Now this is the single most common mistake that I see being made in this context. Let's look at why it is wrong. The sentence now shows us a man sliding a gun about on the surface of a desk - but that is not what the author meant - the sliding was to be how the gun got from his hand to the desk - he slid it on. Do you see how using the wrong word has completely altered the sense of the sentence? If we say, more correctly, "He slid his gun onto the desk," we see, as we are meant to do, the man sliding the gun onto the desk.
Similarly with "in" - I very often see sentences like "She walked in the room." This is particularly rife among American writers, and of course their spoken dialect allows this usage in casual speech, at least among certain groups. But in narrative, again, it distorts the meaning of the sentence away from what the writer intends. "She walked in the room" is equivalent to "she walked in the garden" - i.e. she was in the room walking about. But when the sentence is meant to depict the person entering a room, it is misleading, and if despite this the writer's true meaning is evident from the context, it looks rough and illiterate.