Can you tell me how many typing mistakes you made in your live? Probably, countless times a day, right?
Well, don't feel bad! It's absolutely normal, every human being makes mistakes. What differentiate us from other animals and machines (and QTP, I might add), is the fact that we learn with our mistakes and evolve towards excellence.
I know you saw that coming, and you are correct in your expectations: the datatable pane DOES NOT allow you to undo editing. No Ctrl+Z, no right-click menu option, no nothing. Bear with me, you must have a copy of your precious datatable (if that's the case), cause if you accidentally type anything over a filled cell, I'm sorry for you!
Workaround: (1) be very careful to not mistype inside the datatable, (2) have periodical backups of your script, or (3) enable version control (in Quality Center or any sort) for your script. Other than that: I'm sorry for you!