01234567890
AAAAAAAAAA
AAAAAAAAAAA
==========  Offset 0 -- 0 mismatches.

01234567890
 AAAAAAAAAA
AAAAAAAAAAA 
 ==========  Offset 1 -- 0 mismatches.

01234567890
CCAAAAAAAA
AAAAAAAAAAA
xx========   Offset 0 -- 2 mismatches.

12345678901
 CCAAAAAAAA
AAAAAAAAAAA
 xx======== Offset 1 -- 2 mismatches.

## 806 other 2-mismatch matches your code fails to find

12345678901
AAAAAAAATT
AAAAAAAAAAA
========xx   Offset 0 -- 2 mismatches.

12345678901
 AAAAAAAATT
AAAAAAAAAAA  
 ========xx Offset 1 -- 2 mismatches.