When you think one word, but write another, similar sounding word?

Question

If you are writing or typing and you are thinking of one word, but then type another word made of the same phonemes, what is that called and what are the linguistic and /or psychological phenomena behind it?

I'm actually having trouble coming up with examples, but it happens to me quite a lot. I might want to say something like "You need to use shields," but since the /z/ from "use" assimilates into "shields," I might end up typing "You need to you shields."

(This is a rather poor example, and I will edit the question if I think of another one, but this happens to me, and happens to a lot of other people quite often.)

Answer 1

A term I know from psycholinguistics is "phonologically based lexical selection error".
That means, when looking up the words you need in your mental lexicon, you already have the almost appropriate phonetic form in mind, but then accidently choose a word instead that is phonetically very similar (i.e. differing in one sound, as in your example), but semantically misplaced.
Other examples would be He has a new commuter (instead of computer) or The noun comes after the proposition (instead of preposition).

A lexical selection error in general is when you erroneously pick a wrong word, which is, however, a valid word in your language.
Other sub-types of such lexical selection errors beyond phonologically based ones are semantically based lexical selection errors (one prominent error is choosing the exact antonym of what you mean, e.g. It's very cold - err, hot) or errors involving morpohological stranding (e.g. They are Turking talkish).
Interestingly, many lexical selection errors still come along with correct grammaticality, as in the example I just mentioned: Although the lexical items themselves are misplaced, the inflection (i.e. -ing and -ish) is syntactically still at the right place - which is an indicator for mental lexical retrievement being a different process than building up the grammatical structure.

Such lexical selection errors are different from pure phonological errors, in that they involve words that are still part of the lexicon (just semantically misplaced), where you already pick the wrong word for your sentence before starting to pronounce it. Pure phonological errors would mostly be something like he gave the goy a book (instead of the boy) or lunder and thightening (instead of thunder and lightening), were you realise certain phonemes the wrong way but without that resulting in a phonetically similar, but semantically different word which is not a valid word of the language.
So the place where this goes wrong is a different one; it is not during the process of retrieving the words from your mental lexicon, but rather during the pronouncitation process, after you have already chosen your words and built up the grammatical structure.

So to summarise, I guess what you mean is a speech error named phonologically based lexical selection error, where you don't just mispronounce things but rather make an error in getting the right words from your lexicon, by choosing a phonetically similar one that you then pronounce, often even inflect correctly, but that is just semantically not any meaningful in the context you want to use it.

Edit:
A comment on amls answer (I would have commented, but don't have enough reputation):
I agree with you that there goes something wrong during monitoring which might be called "momentary aphasia", however I think this is not adressing the problem exactly: What OP wanted to know is type of speech error this is, i.e. what goes wrong in the first place to even produce such a sentence - not primarily why this might in some situations not be monitored and repaired.

Answer 2

I would call it 'momentary aphasia'. The brain has different modules for input, memory, awareness and output; and all are highly parallel. The awareness may miss that the output was distracted by a similarity and went on a tangent.

Answer 3

"Freudian slip" e.g

Jimmy Kimmels 'clip slip'

Or one which can be reproduced here

'For seven and a half years I've worked alongside President Reagan. We've had triumphs. Made some mistakes. We've had some sex... uh... setbacks." -A Freudian slip by President George H.W. Bush

Also called parapraxis and includes both slip of tongue and of the pen. The latter being termed lapsus calami.

The source may be psychological but is not limited to it...in your example naturally you feel that sheilds will be useless and it is the bearers who will bear the brunt.

Answer 4

Your question obviously directs to homophony, or homophones - reread the title: "When you think one word, but write another, similar sounding word?", although the reasoning given points to a totally different direction, as you mention same phonemes - which is entirely a different proposition. You're pointing to a concrete omission, rather than an "assimilation" of the sound into the next word. In the given example you lose the meaning entirely as the "assimilated" example makes no sense anymore. You have to pronounce and write both words "use" and "shields", otherwise you'd have to phrase it as "You need youR shields" - where the actual verb "to use" (the action) is subliminally understood and needs not be inserted.