Oops, I deleted it again
We've all deleted stuff we didn't want to, and you probably got it back by mashing the good old CTRL+Z key combo until you've got back the objects you deleted. This however undoes anything you did after deleting these objects. So for example if you deleted something and then went off and did some other work, to get back the stuff you deleted you'd probably end up undoing all that work.
Whilst "oops" is an apt comment, I'm actually referring to the OOPS command. This will reinstate the objects removed by the last ERASE command, and is really handy in the right situation.
This one isn't just for mistakes either. There have been many times that I've deliberately used OOPS - for example, if I had certain objects on top of each other I could select the top objects, erase them, move the objects underneath somewhere else, and then invoke OOPS to retrieve the deleted objects.
I hope knowing this command helps you as it has helped me!
P.S. Oops, I'm accidentally going to slip in a reminder to subscribe below... ;-)