Some files on my site have similar names. For example P100.JPG and P100.jpg.
In MySQL nonbinary string comparisons are case insensitive by default.
Therefore sometimes evalution linked node going wrong.
There is simple patch for 6.x-1.7 (and current 6.x-1.x verison too) that solves this problem on my site. I've just added word "BINARY" in 3 SQL comparision expressions. Hope this helps someone else.