Press ctrl-alt-L + enter to reformat the current file. Press Ctrl-Z – you can undo.

Press ctrl-alt-L + enter to reformat the current file. Press ctrl-alt-L + enter again, which is a no-op action since the file is already reformatted. Press Ctrl-Z – you get a "cannot undo" dialog complaining about an empty list of problem files.

Related to http://jetbrains.net/jira/browse/IDEA-9528 but I don't know if it's the same issue since it happens under different circumstances.

(By the way, the message should probably be something like "The following files affected by this action have already been changed". And you should say which action you mean, or perhaps say something like "affected by the action you are trying to undo".)