Thanks milandro.
I don't remember which is the option you mentioned about shooting independently wether the focus has been achieved. In any case, I'm pretty sure I got focus before shooting. Is there any way to check where the focus points are in the photo? (I remember Canon's software had that)
None of my examples were taken in continuos shooting mode, since I found this issue some time ago I usually take multiple photos when I consider necessary, that's why some of them look almost identical.