some mistake / some mistakes

English as a Second Language English Vocabulary, Grammar and Idioms
1

[color=blue]Why am I under arrest? There must be some mistake.

Cambridge Dictionary

Hello,
“mistake” in all of the dictionaries is defined as a countable noun. Then it should be some mistakes not some mistake.
Would you please clarify the example to me?

Thank you.

2

‘Some’ can refer to a singular noun… as seen here.

3

That is, when the person, place or thing is neither known nor specific/certain as in:
He is living at some place in the UK.
I remember having read that story in some book.