Sometimes while editing a job, you'll notice that when you type text, the cursor begins to delete the text in front of it.
This is because you're in Overstrike (OVR) mode. This usually happens because the Insert key was accidently pressed. You'll see you're in Overstrike mode by looking at the bottom of CATalyst and seeing OVR in black text.
Simply press the Insert key again to turn off Overstrike mode.