Proofreading is the least favourite part of my job. Not surprisingly, it is also the most important. Of course, the spellchecker on MS Word points out the glaring problems with grammar and spelling (I absolutely cannot type Mpumalanga correctly – Mpuapangana, Lysgfkjfg, dammit). Then, I run it through the Grammarly check too, which is even more specific and (sometimes) pedantic.
But, the real challenge comes in picking up the errors that look ok to the spellchecker. They’re particularly difficult for the writer because we often read what we wanted to say, not what we actually typed. Here are some common errors that can totally decimate the value of your writing, without even a trace of the red squiggly line.

Grammar Issues to be Aware of
Numbers
Nope, spellchecker has no idea that you’ve been talking about 1992 for the past six paragraphs and now you typed 1993. Keep notes, if you need to.
Tense
This can be as a result of a lack of focus or typing muscle memory. It’s a really easy mistake to make but makes the sentence read so awkwardly. E.g. “She opened the shutters, drank in the cool night air, and tried to forget her shattered nerves. Grant walks in with all of the finesse of a gorilla.”
Plurals
This isn’t always as easy as it sounds to get right. If you talk about “a variety”, it’s singular (there’s one group of different things). So, your sentence should read, “There is a variety of water sports on offer”. It would also be correct to say, “There are many water sports on offer”. Focus on exactly what is singular and what is plural in your sentence for better grammar and readability.
Homonyms
These are words that sound the same but are spelt differently.
Their and there
Two, too and to
Pause, pours, paws and pores
I recommend reading the piece backward to pick up these sneaky little critters because, although they’re spelt correctly, they may be wrong in terms of grammar standards.
Compound Words
Your spellchecker isn’t likely to spot compound words that have been split unnecessarily. These include words like:
backwards vs back wards
teapot vs tea pot
bathroom vs bath room
Words Left Out
This is tricky to pick up but glaringly obvious for a reader. Leaving out words like “the”, “and”, and “or” is common and frustrating.
Incorrect Words
These make most avid readers twitch, but your spellchecker doesn’t even flinch. Heard of someone ordering an “expresso”, people who love all animals “excepting” rhinos, “irregardless” of the fact that they’re endangered? I’m feeling woozy already.
Possessive Pronouns
Spellcheckers aren’t likely to pick up that you’ve used “her” instead of “hers”, so make sure that you check your pronouns with a fine-toothed comb.
Names
Most names are accepted by spellcheckers. They don’t care if you’ve been referring to “Stephen” and now you’re (unconsciously) opting for “Steven”. But, any Stephen reading your content is going to notice. So will his mother.
By keeping a tight rein on your grammar, you give your content (whether creative or business-oriented) an added edge that really can’t be quantified.