Proofreaders today are usually freelancers. They look for errors in typing, spelling and punctuation but do not normally make substantial changes to a text. However, most proofreaders also offer copy-editing services, which involve more detailed intervention, such as improvement of sentence structure, even reorganising entire paragraphs when discrepancies occur between actual and intended meanings. It’s important for an editor to strike a balance here though; all writers are different, and their work is unique. Over-editing can introduce mediocrity, removing the fire from a piece of writing in the process of making it ‘correct’. The focus of both proofreading and editing, therefore, should be on enhancing the quality of a document, making it as good as it can be, without silencing the voice of the author.
Today, technology such as Microsoft Word is useful for reducing the risk of documents containing non-words; e.g., tomorow instead of tomorrow. Errors such as this are easily missed, especially for self-checking authors with pre-existing context-related expectations, as we often mistakenly see what we expect to see rather than what is actually there. It is not difficult to understand tomorow as meaning the day after today, but let’s consider sentences containing words with jumbled letter strings. There is some evidence that wrod oerdr dseno’t mtaetr, the olny iproamtnt tihng is taht the frsit and lsat ltteers are in the rghit pclaes.  Despite looking, at first glance, like gobbledegook, the sentence is, nevertheless, understood by many people, which is evidence that the brain doesn’t necessarily register each individual letter in a word but, perhaps, engages in some kind of shape recognition.
Fortunately, spell-check software would clearly have a ball with such a sentence, but correctly spelt words used in error are unlikely to be detected in an automatic spell-check.
For example, only one letter separates, ‘We are now able to offer you the position’, from ‘We are not able to offer you the position’. And what if a sentence does, literally, hang on a letter: ‘The defendant has pleaded guilty, and we are not able to propose a community-based sentencing option’?
Checking such sentences can be tricky, especially when editing our own work, because, as stated above, we can wrongly identify a word that we expect to see rather than processing the individual component letters.
Context is everything, and we don’t normally need to be so precise in order to be understood. How often do we read, ‘Remove film lid and place in pre-heated oven’? I must admit to promising myself that I shall respond to the company, complaining that the film lid is charred and my food still frozen; but, of course, I never do. Taking a red pen and correcting the instructions on the packet before discarding it has had, so far, to suffice. Sometimes, it’s necessary to take a prescriptive approach to writing, such as in science, medicine and law, for example; but at other times, such as in creative writing, it may be prudent to value communicative skills – enthusiasm, energy, sentiment – over correctness. There’s no consensus of opinion over this, of course. Why else would someone create the Apostrophe Protection Society or, in fact, its counterpart, the Kill the Apostrophe movement? I really don’t want to become OTT about a few punctuation marks, or to become so disturbed that I share Lynne Truss’s (2003) zero tolerance approach to punctuation and find myself, like Lynne, ‘quite stranded’, feeling ‘cheated and giddy’, and in very bad cases ‘fall over’ whenever I encounter a missing comma.  No, I prefer to consider myself objective, dispassionate and able to take the middle road.
And now, for lunch; has anyone seen my red pen…?