Subclausal negation and clausal negation: "not an uncommon mistake" vs. "a not uncommon mistake"

Question

I'm confused about distinguishing the meaning difference in scope of negation.

1. This is a common mistake.
2. This is an uncommon mistake.

I know that (1) means the mistake occurs often, and (2) means the mistake rarely occurs.

However, in these sentences,

3. This is a not uncommon mistake.
4. This is not an uncommon mistake.

I don't know the exact meaning in (4).

I know the negative item 'not' in (3) goes with "un" in "uncommon". Therefore, this sentence can be paraphrased as " This is somewhat common."

But, I don't get the meaning (4); however, I acknowledged that the negation scope (not) goes with "is".

Does sentence (4) have same meaning with (1)? (I just erased two negative items "not" and "un")

Or is there any other interpretation about (4)?

Answer 1

Both of your examples #3 and #4 mean more or less the same thing. #3 is just a different and less common style that sometimes works better in a longer sentence, possibly when comparing with other negations. For example:

She is not often sad (so it's unusual to see her sad today)

She is often not sad (but not happy either).

The reason is that it shifts the negation from the adverb often to the adjective sad, not-often vs. not-sad.

In a similar way your example shifts the negation from the adjective to the noun. The first says the mistake is of type not-X, while the second says it's not a type-X-mistake. As I said, it's a difference that, in this case, makes no difference in meaning, but might in a different context:

This is a not-uncommon procedure (so we should have nothing to worry about)

This is not an uncommon procedure (but it's not a particularly common procedure either).