Problem: When you’re trying to crop an image using the Crop tool (C), your cropping border tries to snap to the edges of your document window. This might also be happening when drawing large Marquee selections as well. Solution: Press Command-Shift-; (PC: Control-Shift-;), which is the shortcut for turning off this snapping. The only downside is it turns off all snapping (like Snap To Guides, Snap To Grid, etc.). If you just want the Crop snapping (or Marquee snapping) off, go under the View menu, under Snap To, and choose Document Bounds, and your tools will no longer try to snap to your, well, document bounds.