In our virtual world, 'Control Z' is not as powerful as it seems. Our Email is archived and backed up to other systems. Gmail recently came out with a limited message unsend feature, but this only works for a limited period after hitting the send button. Facebook posts can be shared and propagated pretty soon after you have posted and even if you retract a post, it lives on in some archive.
What does this mean in a NoSQL world? NoSQL databases are powering the internet these days, and though it is not required these data stores often contain denormalized data where data facts are often stored in multiple places. For instance a user name may be stored in a user profile and also in all that user's posts, comments and likes. Let's say a user gets married and as a traditional person she changes her last name. Then all her posts, comments and likes need to be updated. SQL databases usually solve this problem by storing the name once and using an immutable primary key for the user that is stored in the posts, comments and likes. Then the two tables are joined to show the name. NoSQL usually does not join its objects and hence the need to denormalize or repeat the data. This makes 'Contol Z' much harder and reminds of us that we do live in a world where actions leave a mark and those marks are hard to erase, and we have to live with the smudges that life gives us.