knife problem

Hallo

Im pretty new to blender and am trying out tools to learn about it. im trying to make a frog character and want to cut out a mouth. but i can select ok with the knife tool, but once i hit enter, its selects totally random edges, in different place[ATTACH=CONFIG]216069[/ATTACH]
than were i selected originally. Photo one is screen shot of were i want to select and the blend file under is were the knife tool has randomly selected.

[ATTACH]216070[/ATTACH]

The only way i found the knife to cut without this bizarre random cuts is to disable those buttons in the modifiers :
http://i.imgur.com/pv1yRjF.jpg

Disable 1st the one on the Subdivision Surface modifieir, then disable in 2nd the one in the Mirror modifier.

Then the cut will work.

To me it sounds like a bug, you should really report it on the bug tracker so the developers fix it if they can before the next Blender release (that is soon).
http://projects.blender.org/tracker/?group_id=9&atid=498