I have a lot of meshes. I selected one mesh then pressed K I clicked on the face to start the cut but when I click again it cancels it. this only happen to one mesh,a cube .
Hmmm that does sound strange. . . .
sshanr Are you using different mouse clicks when using the knife tool perhaps?
The knife tool only accepts the 'active' click to make cuts. If you make an inactive click then it cancels the knife tool.
For example: I am a left-click-to-select kind of guy. So when I cut with the knife tool I must use left click to make cuts. If I right click then it cancels the knife tool (even after making many many cuts with left click!). When you are done with a cut you should press ''Enter' on the keyboard.
Does that help?