Always End With 1089
Add two number together and always end with 1089.
Here is How:
Pick a three digit number. The three numbers used must be different*. i.e. 123 
Reverse that number. 123 becomes 321 

Take the smallest three digit number from the largest. 321  123 = 198 
Take the answer and reverse that number. 198 becomes 891 

Add that number to the answer of the subtraction. 891 + 198 = 1089 
The answer will be 1089! 

So Why does it do this?

A Little Footnote
But I ended with 198 not 1089 ... !
This happens if you pick a number like 546.
546 goes like this: 645546=99, then 99+99=198.
BUT if you assume that 099 is a 3 digit number, you will get:
990099=891, and 891+198=1089
*If you don't pick three different numbers then this might happen:
343343=0, 0+0=0