When using the crop tool at 100% zoom, Photoshop incorrectly crops off the top row and the left column of pixels, relative to where you clicked.
Here is a video: http://youtu.be/uvmWJmRomqc
This is on a Mac running 10.9 and Photoshop CC 2014. This bug has been around for about the last three versions of Photoshop, if I remember correctly.