'Some or sum' is a common grammar mistake because 'some' and 'sum' are two completely different words with different meanings and usage.

'Some' is an indefinite pronoun used to refer to an unspecified amount or number of something. It is used to indicate a quantity that is not specific or exact. For example:

- I have some apples.

- Can you give me some water?

'Sum', on the other hand, is a mathematical term referring to the total or the result of adding two or more numbers together. It is used in the context of arithmetic or calculations. For example:

- The sum of 5 and 3 is 8.

- What is the sum of these three numbers?

Confusing 'some' with 'sum' can lead to misunderstandings and incorrect usage in sentences. It is important to use the correct word depending on the intended meaning.