There must have been some/a mistake.

[GeneD]

There must have been some mistake.
There must have been a mistake.

Is there any difference in meaning between the two sentences?

 

[J&K Tutoring]

Your post has been up for quite some time with no affirmative response. I suggest you take that as meaning there is either:

a. No meaningful difference. OR
b. The difference is difficult to define.

 

[emsr2d2]

In BrE, we tend to use "some mistake" only in the present tense.

There must be some mistake - I can't see my name on the invitation list for my friend's wedding.
I didn't win the "Best Actor" award this year. There must be some mistake!

Effectively, it means the same as "I think a mistake must have been made".