Identify the incorrect number in the following series:
27, 26, 24, 21, 18, 12

Let's investigate the difference pattern between consecutive terms of the series:
* $26-27=-1$
* $24-26=-2$
* $21-24=-3$

The subtraction sequence is decreasing by consecutive integers: $-1,-2,-3$.
Following this logic, the next subtraction value should be $-4$:

$$\text{Expected term}=21-4=17$$

Let's verify the next step with the expected term 17:

$$\text{Next term}=17-5=12$$

This matches the final term (12) perfectly.

Therefore, 18 is the incorrect term in the given series; it should be replaced by 17.