Run the mongo shell, type "cf" and hit tab. You get an auto-completion suggestion of "cf_1_f_x)
{shellAutocomplete(x)}(".MongoDB shell version: 2.1.0-pre-
connecting to: test
> cf<hit tab here>_1_f_x) {shellAutocomplete(x)}
(
Presumably, some leftover cruft from the shellAutocomplete() code, but I haven't looked into it yet. Tested on Windows and Ubuntu.