error vs misunderstanding vs fault vs slip

[Saya Otonashi]

Synonyms Lesson, Advanced Level

ESL/EFL Test #112 "Synonyms for error and fault", question 9

It is written here that I have a sister but there must be some ......... because I am an only child.

(a) error
(b) misunderstanding
(c) fault
(d) slip

Synonyms Lesson, Advanced Level

ESL/EFL Test #112 "Synonyms for error and fault", answer 9

It is written here that I have a sister but there must be some misunderstanding because I am an only child.

Correct answer: (b) misunderstanding
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when can we use error, misunderstanding, fault and slip?

Saya Otonashi

[Alan]

Hi,

Similar words all suggesting something is not right. 'Misundersanding' suggests confusion over information caused by bad communication - Fault is often a technical failure as in: There is a fault on the telephone line and I can't ring anybody up. 'Slip' is often a minor mistake as when you spell someone's name the wrong way. My surname is Townend but many people spell this Townsend.

Alan
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English as a Second Language
You can read my ESL story Womens' Day

[Andrei]

Hi Alan!
I've got a little irrelevant question but still:
I was told that we should use "an only child" if there's a possibility of Parents' having another child and we use "the only"child" if they can't. I just want to check whether it's correct.
Thanks in advance