Keystroke: Ctrl + z
This feature, sometimes known as "Go back", allows you to "undo" various operations, such as a Search & Replace, as well as incorrect typing.
It should however be noted that the feature only works during the current open session of a document within DBT. Once you close a document, regardless of whether or not you have saved it, the feature will not be available.
You should use Undo with care, since there is a limit of 100 operations. However, "Replace All" does count as one.