The 'undofile' is a great feature, and I use it a lot. However, one annoyance is that it's very easy to accidentally undo changes that I did the last time I opened the file; which may be 2 minutes ago, an hour ago, last week, or a year ago.
So what would happen is that sometimes I would accidentally undo changes from ages ago.

This plugin fixes that by showing a warning.

I originally posted this as an answer on vi Stack Exchange, which also has some other solutions to the same problem.

See :help undofile-warn for the full documentation.