When using some of the fancy command line editing shortcuts, such as
ctrl-u, you may end up erroneously changing or deleting part of the
current command. Retyping it may be a pain or impossible if you've forgotten
exactly what was changed. Fortunately, bash's command line editing has undo
built in. Just hit ctrl-_ a couple times to get back to where you want to
be.
h/t Chris Erin