One of the errors I see most often in the last few years is that of randomly concatenating words which should be kept separate.
The flagship of this flotilla of doom is the egregious 'everyday', which rears its ugly head in just about every page of Facebook and every el cheapo, do-it-yourself advertisement. From this piece of nastiness, the disease has spread, giving us such gems as 'alot' and 'backseat'.
Now, 'everyday' and 'backseat' are, of course, perfectly acceptable words ('alot' is not a word at all); the error lies in their inappropriate use in contexts where what was called for was 'every day' and 'back seat'.
Why is it so? I hear you ask. It is so, Grasshopper, because concatenating a pair of words into a single word changes them into a different part of speech.
Consider the phrase 'every day'. This modifies a verb, so it is an adverbial phrase, and functions in a sentence just as an adverb would do.
For example: Rover was taken for a walk every day. Rover was taken for a walk frequently.
Now, when we concatenate the words, we have 'everyday'. This used to be a very common word, but has fallen rather into disuse in recent years. It is an adjective, and means 'ordinary'. You can see the relationship between the two expressions, stemming from the fact that ordinary things tend to be things that happen every day, but they are quite different.
EG: On weekdays Rover wore his everyday collar, but on Sundays he was put into his diamond one. On weekdays Rover wore his ordinary collar, but on Sundays he was put into his diamond one.
By substituting the alternative expressions in the example sentences, we can see how inappropriate it is to substitute these two expressions for one another.
Rover was taken for a walk ordinary. On weekdays Rover wore his frequently collar.
See how stupid it looks, and how devoid of meaning?
Now, let's take a look at 'back seat' and 'backseat'. As discussed above, these are two different parts of speech, although not the same parts as with 'every day' and 'everyday'.
'Back seat' is a noun phrase, consisting of a noun ('seat') and an adjective ('back'). 'Backseat' is an adjective. Let's put them into sentences.
1a: When Rover was taken to the vet, he rode on the back seat.
1b: When Rover was taken to the vet, he rode in the back of the car.
2a: Rover's owner was a backseat driver, constantly giving advice to the chauffeur.
2b: Rover's owner was an interfering person, constantly giving advice to the chauffeur.
Now, let's swap them over.
1c: When Rover was taken to the vet, he rode on the interfering.
2c: Rover's owner was an in the back of the car driver, constantly giving advice to the chauffeur.
2c can sort of work, with the addition of quotes around the bolded part, because of the close relationship between the noun phrase and the adjective, but it's awfully clunky, isn't it? You can tell it's not the best way to put it. And of course, without the quotes, it loses its sense. 1c is quite impossible.
There are almost as many errors of this kind being made today as there are illiterates on the internet, but the core problem is always the same, and stems from the fact that you cannot substitute one part of speech for another.