Ok, so sometimes when making drastic changes in your model, like, say, modifying a part of your car model, You may change the hood around, then say “I made THAT ABOMINATION?!?!” Looking over the newly-modified hood, you notice it looks more like a deformed piece of bacon. Trying to undo the change, you hit command-z about 30 times and then you cannot ‘undo’ any further. This is a problem, because your undoing does not go far enough back to cover for the ugly hood. So, if this is at all sounding familiar to you, there is hope!! You may notice in your Blender interface two boxes containing 10 squares each, these are called layers. And they are critical to this tip. To change a part’s layer you select the desired part then hit M. The 20 boxes show up at that point, and when you click on one, the object disappears. Now you will see a dot in the layer you just put your object in. This is not a fancy trash can, there is a way to get back to your model. you can click on the box of the layer you want, or you can click one of your keyboard’s numbers. From there you can move it back or do all the stuff you would normally do on layer 1… I could from here go onto a lengthier discussion of the layer and the part it plays in modern Blender modeling blah blah blah, but, for the sake of my typing fingers and your interest level, I will move on 🙂 So, back to the deformed hood- sorry! Nothing I can do about that! Unless you hit the recover auto save button or something… but, anyway, this ‘accident’ could have been avoided had you used the layers as backup layers. You may want to do this periodically: as you go through your modeling, every now and then select everything, hit shift-d then move it to the another layer. Select different layers as you continue to back-up your model. Then you can revert back if you need to avoid any unwanted:
P.S: With my models, I usually do the last layer as my first backup layer, 2nd to last next, 3rd to last etc.